I'm just wrapping up a 6 cyl to 383 swap in my 68, hoping to fire it up tomorrow. A couple answers . . .
Motor mounts - different, replace both frame and engine mounts.
Wiring harness - should be no problem. Oil pressure sender wire will be on the short side, coil wire will be long, everything else was perfect.
Pulley and brackets - you'll need them all. Try to find them in a set from a donor engine or you'll wind up with an anurism like me trying to mix and match aftermarket stuff.
Radiator - you'll need a bigger one. 6 cyl cars came with a two core, V8's need a three core or larger.
Tranny - 6 cyl automatics are lighter duty, but work with mild V8's.