You talked to someone at Summit who does not know what he is talking about. SCE specifically specs the 4235 gaskets for 90-93 460's. I used them on my 96 and they fit just fine. You can look for yourself at scegaskets .com.

 

A big 10-4 on that, I remember specifically I had to tell summit that I had a 460efi 95 with square exhaust port heads. I know they had them 2 years ago and that they have them now. Don't give up!

As for the engine shutting down at 90 then now its 80mph, That sounds like something else wrong, nothing to do with the exhaust you just put on. Mine would go past the 90 mark with stock exhaust, now it goes maybe a little faster with headers and dual exhaust. But the thing is, it would not shut down, it just would not go any faster of a top speed. Not sure on that one, but I dont think its the new exhaust. I added a MSD ignition box and it has its own little computer shift patterns I think, now it lets me use overdrive at top speeds if i set it at 4000 tac Chip thingy. And I go way above and beyond 90mph.

 

Well it's not that the engine shuts down per say. Something stops the truck from accelerating anymore like it has a govenor on it but you can feel the truck kick down hard. Now on my way home from work this morning I hit an easy 90 mph with no shut down but I got the check engine light back when I start to slow down. I think the computer can't handle all the preformance mods I'm doing. I had a few people say that I need to invest in a chip or reprogramer. What should I go with?

 

In your first post you said emissions where strict there? So that means you kept all your smog pump crap installed? The stock manafolds have hoses that connect to the Smog pump system. Maybe they connected something wrong or there are two tubes that goto the cat converters for air supply. What did they do with those? Check the EGR valve connection to if you have one. Maybe your computer sences that one of these is not right.

Also with mine when I put headers on, There was a negative wire to the battery connected to a part on the old manafolds, we left this disconnected and it caused me electrical problems till we found it again and grounded the wire on the block. If I recall right the check engine like came on while this was disconnected along with all my gauges going crazy. Check to make sure that the exhaust shop grounded this wire somewhere. I dont have emissions here yet so I took all the smog stuff off and they capped the cat converters on mine.

 

All of the emission equipment was reconnected. The only thing I'm not sure on is the ground wire your speaking of. I'll have to check that out. It seems like every day the truck acts diffrent...

 

The o2 sensor. Where is it? is it located about where the stock one was? And is it plugged in? Did the exhaust shop accidently damage it when they put it back in? That might give your computer some strange signals if its too close to the motor or too far away from it. If I think of any thing else I'll post it

oh, you said all the smog stuff is still there, Tell me how they connected the The smog stuff to the headers. there has to be a way that the headers are sending puffs of exhaust to the smog pump for it to smogginate or whatever it does like the manifolds did b4.

I know this is frustrating, but your setup sounds exactly like mine with the exception of the smog pump still being active. Thats only why I keep asking about the pump.

 

All the smog pipes were welded into the headers and high flo cats. Actually the headers are street legal ones and had the input for the pipe that goes to what I beleve is the EGR. I replaced the o2 sensor myself when I got the truck back from the shop. It's located smack dab in the center of the "x" pipe so it will read both sides of the engine. I disconnected the batt for about 30 seconds so the puter could reset but now people are telling me to disconnect it for 10 min and with the headlights on. what the hell does the headlights have to do with the puter???....lol

Again last night I had her up to about 95mph before she stoped going and again this morning she was konking out at about 80 or so. I swear the truck is possesed!!!

 

There at the o2 sensor! It is located in the x-cross over to both sides? That could be it, you might want to ask around but that to me sounds like it could be it. You need to have it about where the factory one was probably a little be closer to the engine where it can read just one pipe not in the middle of both. Right now you have got sort of a whirlwind of exhaust going through the x-over. and the faster you go the more exhaust whirlwind it creates. Having all that go to the computer prolly is messing it up at top speeds telling it there is too much fuel in the exhaust or not enough, what ever the o2 sensor does. Just having a whirlwind around the sensor is prolly messing up the readings its suppose to take. It is reading both sides right now, so in therory, the o2 sensor is reading maybe double the fuel in the exhaust so to correct this the computer prolly feeds less fuel to the engine when you start going fast = it will konk out at 80 - 85 due to not enough fuel being injected into motor to go any faster. Read up on the o2 sensor, you might have to have them reweld it on the passanger side pipe, well in fact you will have to. Tell the exhaust shop this and see what they say. That sensor has to be just right, or near where the factory one was, in order for the fuel injectors to function properly. You wouldnt notice this at low speeds cause there is not enough exhaust going throught, but when you go fast then it downshifts it creates a big whirlwind, maybe related to your check enging light coming on when you slow down. you might want to do this with the o2 sensor first before investing in $$$computer chips. Sorry for the long post but this to me seems like that might be whats happening. With all this electronic crap I miss the carborator days with springs and pins