Heads

If it comes on good at 2500-3500 but then flattens out you could just be running lean. Have a look at those plugs and see what they look like. If you have a chassis dyno in your area that would be the best way to get it maximized. May cost a couple hundred bucks but it's well spent.

BTW: Is the distributor stock? If so the advance curve needs to be modified to limit total timing so you can run more initial timing and have it all come in faster.