Buddy of mine used late model GM V6 pistons on his slant. I'm not sure on the bore size. I do know they were way cheaper than the slant pistons.
Buddy of mine that built the motor checked his files.
For the slant that's bored .100
2.8 Ltr Chev piston. The bore size is 3.5039
Use 198 rod to make up piston height
The Chev pin bore is .905 and the slant is .901. You'll also have to bore the pin bore .004.
83-85 173 V6 part number 464P. Bore size is 3.5078. Using the 198 rod and boring out the pin bore you shoud have a deck height of .020. Shave the head accordingly to get the desired CR. You should end up with a slant of approximately 238 CI