Mini starters can be expensive. And some of the gear reduction aftermarket stuff can be noisy. I had the same issue you are having, hard to crank over when hot with the 351W in my 1969 Mach 1. This was many years ago, I went to the parts department at my local Ford dealer, the part number changed a few times, but I ended up with a late model permanent magnet gear reduction mini starter. It solved all the hard starting issues.
If you have an automatic trans, I think a starter for a 1995 or so F150 with a 5.8L and automatic trans will fit. Those are a PMGR mini starter and you can get one from a parts store. Just do a close comparison before installing it, especially the drive gear.