I think the springs i had in my edelbrock heads had the same spring rate of 123lbs and based off what my machine shop told me was that the cam that was in it it needed heavier springs because the owner prior didnt know what he did but he bent every single valve because the valve wouldnt close fast enough
You should be able to shim them.
Gt40x are actually pretty decent heads.
But my advice, take them to a machine shop and have them valve jobbed, at the same time have them shim the seats to 130lbs. The ford valve job on them isn't that great, taking care of the seats and doing a 3 angle valve job and bowl cleanup adds quite a bit of power.
While i don't know all the math and calculations involved, i know for a fact my buddy had all gt40x and y's seat pressure set to 130lbs.
He also required all heads that he installed to be valve jobbed, whether you wanted it that way or not, he said that the heads themselves, the valves, seat pressure and bowl jobs varied so much, that the $200 was great insurance.
Gt40x are actually pretty decent heads.
But my advice, take them to a machine shop and have them valve jobbed, at the same time have them shim the seats to 130lbs. The ford valve job on them isn't that great, taking care of the seats and doing a 3 angle valve job and bowl cleanup adds quite a bit of power.
While i don't know all the math and calculations involved, i know for a fact my buddy had all gt40x and y's seat pressure set to 130lbs.
He also required all heads that he installed to be valve jobbed, whether you wanted it that way or not, he said that the heads themselves, the valves, seat pressure and bowl jobs varied so much, that the $200 was great insurance.