engine mounts are not the same.
the engine sits taller, so you might need some different sheetmetal.
your trans will not standup to the BBC. 4L80e is highly recommended.
to get ample radiator cooling, youll need a custom radiator as the
OE is too tall. your 1/2 ton frame rails are smaller and narrower so
some enforcment and cutting will probably be necessary.
i know three gents that started to do the same on NBS trucks and
lost interest and money as the fab work piled up...

airdeano