Check all the connections, condition of the wiring ect. How old is the solenoid? Maybe replace that, check tdc to the balancer as best you can with the head on, fab a heat shield out of some flashing or drier duct if all else fails, then buy a starter imo.
I have had slightly harder starting since going to an hei that depends on the initial advance, but it always starts right up hot... And it always starts, but depending on where it is in the stroke it may turn slow in the first turn then it fires, hot or cold. But generally it turns fast. I have a 2 bolt starter and used one for an early 90's auto. Ford only made 2 different 2 bolt starter flanges from the 60's into at least the 90's afaik and the difference is whether it's an auto or a manual. The starter bore of the manual trans bells are slightly larger, but otherwise the bolt centers and the starters look the same. With the 3 bolts ones I think it's the same but the choices are much more limited, esp with small starters like they used on my 92 F150. You have to go aftermarket and pay big $$ for 3 bolts I guess