I have a 90 C3500 with the 6.2L. I don't have the heads for the engine, the shop that took the engine apart didn't send them back with the tow truck. anyways, from the vin it should be a 'J' code engine. I am looking at buying used heads, but i don't know what casting number the heads should have. Can anyone help me out on this.

Since you have spent a lot of money so far I would consider putting 6.5 heads on your engine. You would need new injector lines as well as injectors since the head configuration is different. You might be able to find used lines as well as injectors.

If i were to put 6.5L heads, would i also have to change injector pump, or intake manifold?

The D2 injection pump would still be good, but I don't know about the intake manifold. A lot of people have put 6.5L heads on their 6.2L engines when the heads failed (cracked). I believe the 1992-93 heads are the preferred ones.