I do both wehn I do them. I think for sake of safety, have him build and run it/break in the new engine. But once that's done, you can install it easy enough.
I have complete confidence in my own mechanical skills, but am considering having him do the work so some little glitch I might miss, having been son long since I did a build, doesn't slip thru the cracks, so to speak, and toast a perfectly good engine. This guy does this work all the time, so..............