Why are you going to rebuild motor? What do you mean by "rebuild" ? Is it all apart? Frozen? Tried to start it? Are you "new" to motors and mechanics? or "new" to GMC or ? Not much experience in old trucks and motors?
Are you planning on being a do-it-yourselfer or a having-it-done'er ?
What is your goal as far as type of finished truck?
1. Full original restoration?
2. Good daily driver?
3. Modified here and there?
4. Fix it up a little?
The process of finding a local machinist involves, here say, then possible past customers, then a visit to the shops. Do not leave the truck, leave the motor. Sounds like you are just wanting the motor to be "machined" and you will assemble. Being a GMC, there could be some specific motor internal parts. Don't know if any Chevy stuff fits. Be real sure all the parts are actually available, not just advertised as available. Rockauto may be your best bet. Normally you will have to buy some parts and take them to the machinist to measure. Some parts may have to be specified by the machinist depending on condition of bores and other surfaces.
Lots of motor builder members here. they will chime in.
