Re: HELP! Installing GT40 Heads on a 289 engine.
Just going by what I've read or been told;
stock pedestal rockers may hit the retainers so you'll have to check that.
The FRPP rockers for those heads require the taller 5.0 HO valve covers. They may have a clearance issue with regular height valve covers.
I think the GT40 valve springs are 120 pounds closed/240 open, 1.82 inch closed height, 1.4 open height. If that is in the range of your cam you should be good to go.
Considering all the things that can be involved you'll just have to check your pushrods. I think that is the only roller related issue.
The GT40 heads may have the intake ports lifted. I don't know if they all are that way or they redesigned the intakes at some point. If they are you will have an intake mis-match. You should have enough material in your intake to match that up. The GT40 heads require a specific intake gasket.
1965 289 mustang fastback