Buying an engine from someone that you don't know, or who built it, is always an iffy situation. Are there reciepts for the work? From a reputable rebuilder? Have you seen the engine? If so, does it look like it's been rebuilt, or just painted up nice & pretty? If you think that all is copacetic, a big block will give you big-time low-end torque over the revs that a small block has. A 6.0 or 5.3 will give you comparable numbers, but is a much more complicated swap. These trucks came with BBC's, so the parts for the swap are available. Is the rest of your drivetrain up to the stresses that a BBC will impart on them? If not, a heavy right foot behind a BBC wil certainly find them, right quick! Also, be prepared to be filling up more often. These engines can be pretty thirsty.
