Swap Help please!

[Souths79]

hey everyone, ive got a 79 Sierra half ton 2wd,factory 350 going to a 454. i changed the frame mounts from the small to the big block type and theyre not even close. the sb type are a closer fit than the bb. frame isnt bent and i dont think im that crazy! i can get the sbc mounts to work but the issue there is the engine sits different and the headers (headman) dont come close to the head at all due to dang near the whole header hitting the frame. is there a such thing as a big block clamshell? i assume the were all the same but thats i where a think the prob is. any help would be apprieciated