i have a 73 f100 swb orginaly 3speed .The 302 is from an 80 f100 trying to put c6 that came with 80 motor but will not fit the cross member between where torsion arms bolt to frame is in the way can i cut this and reweld it or is there a trick to installing it?

 

Can you not get it in at all because of the frame member, or when in place, is it hitting that?

Was a 302 originally in the truck? If not, you need to change the engine perches.

And are you trying to install the 80 motor and trans as a unit, or engine first and trans afterward?

There's definitely something you're missing. Mine has a 360/C6 and I've got quite a bit of clearance between the bottom of the trans and that frame member.

 

Your 73 f-100 has radius arms(not torsion bars), I believe. If I'm understanding you properly, you are converting from 3 speed manual colomn shift, to a 1980 small bell C6 automatic....Right? If your truck originaly came with a 302 v8, there are a few ways that I have done with sucsess. 1st. go find a used crossmember from a 73-76 ford 1/2 ton,and install to your truck and transmission.
2nd. Never cut your frame(it becomes very weak). take a piece of pipe 1.5"x.120 wall and build you a nice cross member that bolts to to lower section of the C channel frame, just behind the radius arm brackets. You said (cut and reweld), so you have a welder and torches.You will also need about 6" of 2" x3/16" C channel for the trans mount to be bolted too after you drill the hole in it and weld the C channel to your heat formed pipe/Tube. Attaching to your frame, a couple of pieces of 1/4inch plate notched and welded into place on your new formed crossmember. approx.3"x6" plates. Drill and bolt it to the frame with 2 3/8 bolts,nuts,and washers.
3rd. Using the crossmember you have, since it already has the trans mount that works, cut weld and adapt it to the 73 as you see fit.Just remember that you have electrical harness,fuel lines,and brake lines running down the drivers side frame rails. Clearancing them and not damaging them are critical. Don't forget to use good jack stands to protect your self also, from the truck crunching you and your equipment!!

 

I just did that ,only with a 84 302,in a 79 f-100 . Luckly I had a 79 f-150 parts truck that had a c/6 in it when it was my work truck . The c/6 crossmember drops down a couple of inches more than the f-100 standard crossmember does. So that may be all you need on that end of the project, the motor mounts may need to be changed to something different. My f-100 had three different holes in the frame so the transmission crossmember could be moved forward or backward to where I wanted it. I installed the transmission and engine together,but I had the front clip off of it .Good luck, JIM