87-90 is easiest and requires the least amount of fab (it doesn't matter which year you get, they're all the same with the exception of 24 and 26 spline axles).
85-86 is lightest and requires additional work to strengthen weak areas, some components do not interchange with later year models.
91-92 is heaviest and takes the most work, but winds up being the strongest (provided you strengthen the same areas as all other frames)...you also get the no-link rear shock.

As far as modifying the frame, you need a minimum of these modifications:
Spark plug cutout
RH lowest rear engine attach point needs to be 1" farther out
Wishbone needs to be modified or new downtubes need to be made for the front engine mount
RH frame usually needs some clearance for the kickstarter, my hybrid doesn't but it's also the exception rather than the rule.

That's the absolute minimum, but most people gusset the tubing clusters and use additional tubing to strengthen other areas that need attention.
You'll be putting more stress on the frame because...
Engine is heavier
Engine torques the mild steel frame more
You'll likely be going faster which puts more stress on the frame too