Anyone ripped the c6 out and dropped a 435 in?

is the firewall already prepped for a clutch pedal? basically a bolt in?

any other snags other than driveline lengths?

I did it with a no hydro clutch in my 84. he only hole I had to cut was for the clutch rod to go through the floor. Same x member, either cut a hole for the shifter or get a new trans tunnel cover from the junkyard. The pedals bolt right up. If the 83 had the option for the hydro clutch, the brake booster brace/ plate part of the firewall will have the approriate holes in it. You will need to drill out the firewall under it.
It was a simple swap, it took longer to get done due to the amount of beer flowing.

On my friends 91, Everything was a bolt on.
Get got a new tunnel cover with the shifter hole already cut.
The pedal brackets are a bolt on as well.

different years I konw, but on my 94 that never came with a stick and 351, mine was pretty much a bolt in. i used a zf (hydraulic clutch) but all the hose mounts were there, as well as a cover plate where the MC should be. I used my orig tunnel cover, and just plas'd a whole out.

On an 83, the firewall should be stamped where you need to drill beside the brake booster. My 82 was, my 81 wasn't.
http://www.bigblocksix.com/f100swb/f250/cabswap/drilledfirewall.jpg

If you do a linkage clutch, you may have the hole already there, and just capped:
http://www.bigblocksix.com/f100swb/f250/Untitled-1.jpg

I snagged that plug out of another 82(mine was linkage clutch, swapped to hydro with the 460 chassis,) so, you may already have the hole in place.