I just bought a set and have them on my car and they work really good but change the valve springs before installing them the explorer springs can handle much in the way of a cam--getting float with a stock cam and 1.72's so change them before putting them on.I'm using the trick Flow Kit from summit ($130) seems to me that they are the same as the FMS springs and are just put together with the correct retainers and locks,shims to make installed height work. good complete kit. but yes the gt-40 irons make good power for cast iron ford heads but your compression will drop with a stock bottom end, think its like 9.02:1 so you will be able to run 89 probably if you dont go nuts with the timing and have an aftermarket box.