Percy's Seal-4-Good, There are three different ones depending on which heads you have. 66029,66035 or 66054. Been using them for years they don't leak and they are reusable. I usually spray them with a coat of high temp aluminum paint before I install them. It helps to keep them from sticking if you have to remove them.

 

 

The best gasket won't matter, those are terrible headers. The flange is thin and not machined flat. If you haven't bought them I can suggest some much better ones. Even the Ebay stainless ones are nicer.

 

 

I always used Hookers on the 429's I had so I can't speak to the Flowtechs. But they should be ok. I looked on Jeg's and it said they had 3/8 inch thick flanges. That's plenty stiff if they are straight. Just be sure to retorque them a couple of times after you run the engine.

 

I've used Percy's Dead Soft Aluminum gaskets and Stage 8 locking bolts. Used on small block Windsor engines with Headman Hedders and had good results.

 

Headers are likely the problem, flowtech are junk. Good straight header flanges and a thin smear of orange or red high temp rtf on the head and thin bead on the header flange is my secret. Let it skin for 5 minutes then assemble finger tight. Let it sit for a few hours then torque.