Once you build it, even if it runs great, You own it!
Later on if your buddy ever has any issues he will hold you responsible.

Clutch is slipping, must be when you installed the motor!

Have a vibration in the front end, must be from when you installed the motor!

My wife's a pain in the a$$, Must be from when you installed the motor.

You get where I'm coming from.

Go drive your car and let him have someone with a shop put up with the headaches. its not worth the few hundred bucks he will grudgingly give you because you are buddy's and he figures you shouldn't charge him in the first place.