You could get pistons and most of what you need from Patrick's. Stovebolt and Clifford also sell GMC performance parts. I'm sure others here will add to the list. Search the old posts here for this topic there is a lot of info here. Try to find California Bill Fisher's Books. The little GMC Speed Manual is a good one. There have been a lot of things figured out since these were written in the '50s but most of it is still good. He says that 3 15/16" is max bore for a 270. 4" is possible but it may weaken the cylinder walls. Yours being a '59 it has the D shaped combustion chambers with small ports. The big ports are really only desirable, in my opinion, on a race motor. The small ports work better on the street. With a blower it wont matter any way. If you run that much boost you will have no problem filling your cylinders! On the street I'd think half that would be plenty. You would want a mild cam, stock would work. Your engine should have the stamped steel rockers which are pretty strong. These are by nature not high revving engines,6,000 is a lot! I have a '53 Chevy Pickup with a '57 270. I built it in '78 so I don't remember all I did. It's Bored .030,I think I'll see tomorrow, balanced. It runs a 500 cfm Edlebrock,Fenton headers, and electronic ignition. There was some mild porting done on the head. The cam is the hottest one Clifford sold at the time that was usable on the street. I haven't seen the cam card in years. Chevy manifolds can be fitted to GMCs but it takes some work. My Fentons were actually cast for a Chevy. I would think that your tube headers could be made to fit. That's what we do! I too run a T5 with a '68 GMC Dana 44 limited slip with 355 gears. It'll run 70-80 all day. This one loves to run at 3,000 and I have to think to keep it around 2,500. I hope this was of use to you ,good luck!
