What cam? What heads? What manifold?

[JustOne]

DD,
Go back an look at the post of mine on the first page, the flow numbers for all of the 40 and N series heads are there along with a lot of other heads. The flow data there was done for a head shootout on I belive Westech's flow bench. Anyway, all of those heads were flowed on the same day to get an accurate reading.
What size chamber do you have now? Going up 10cc will only lower your compression about 1.2- 1.5, but you may be able to shave the domes off. If the pistons are used I would scrap them since you can get custom forged pistons for $500.
If you are only looking for a head with 64cc or larger chambers there are many around that are in that range that flow better than the GT40 series head. Remember that the GT40 heads are designed to meet current emission standards and don't flow well on the exhaust side because the have the EGR bumps in them, which is the first thing we always took out to get better flow. You are building a non emissions compliant motor, so don't restrict your search for heads to pieces designed to meet emissions. Remeber your heads cam and intake dictate your HP with the cam and heads being the heart of the combo. Good luck with your build, you picked a great car and having the 289 in there will be cool!