74 Scamp BB swap questions?

as far as difficulty level, I'd say about the same. The small block conversion will be cheaper, because you can use readily available factory parts, K-frame, motor mounts, exhaust, etc.
The big block swap, while not real difficult, will require many custom (expensive) parts. Even a mildly built 318 in a Scamp is a great package,(Mine runs low 12s) & people practically give 318s away. the initial cost of a big block will be much higher as well.