No, 750 is not too slow for a V8 that has 4 power-pulses per revolution. It may be too slow for a 4-banger that only has half that.
I used to run my 292/292/108 even slower.
The limit of slowness is when your rocker arms gall for lack of oil volume. or the engine stalls, lol. To that end, I installed a High volume pump, and re-engineered my top-end oiling system, to live, down at 500/550, in gear (manual trans), and idling down the road .
Now, if your engine does not like idling that slow, and has good even cylinder pressure; that will be either a tuning issue or possibly tight rings/tight pistons.
If you have an automatic, and YOUR engine won't idle in gear at 650, or will but won't take throttle, then I'd bet money that the transfer slots are closed too far. That's a tuning issue.