Swapping pre 74 slant six engine to 75 + engine

We're out in the weeds here with imaginary Canadian restrictions on engine swaps.

OP: there is no practical or legal reason you can't drop the later engine into your '68. Just make sure the oil pan is compatible with your car. If it's not the same, just swap the pan and pickup from your '68 engine into the newer one. While the newer engine's out, it will also be a good idea to service the oil pressure relief valve to make sure it's doing its job, as described in these two threads: thread 1, thread 2.

If you want to keep things simple, you can use the '68 carb and distributor.