[argonaut]

A 61 c10 frame might be wider, but not the k10.

To the OP, check out my Big Ugly thread linked below. I put a 402 big block into my my 66 k10 (basically the same frame) using an engine crossmember from a 71-72 k5/10/20 and transmission crossmember from a 73 K20. I had to cut the stock transmission bellhousing crossmember out. I had to shift the whole assembly forward an inch or so from the factory mount location.

I know another member with a 63 k20 who was able to mount his big block on the factory v8 engine mount brackets and clear the firewall. My conclusion is that these brackets put the block in a lower position, thus clearing the firewall without moving the engine forward.