I was on the Slant 6 forum, but now...

Agree, but if its 727 his existing driveshaft will be too long. If its 904 he should be able to re-use it.
Someone also mentioned keeping your existing scamp wiring. You will just need to lengthen a few wires. Much less work than swapping the whole harness. You can also get a new engine harness for V8. The engine harness are not too expensive like the dash harness is. I dont even bother with salvaging original engine harness anymore, not worth the effort vs the price of a new harness.

Enjoy the Scamp. Its a cool car and a rather less common model .
Going from a slant 6 to a small block, if anything, some wires are too long......not too short. Just fold them over and re-wrap them to the harness farther back.