I have a 72 Chevelle that's hard to start after it's been running. The first start of the day or after it's been sitting for an hour or so is no problem. The issue is once I turn it off, I have to crank it until the battery is almost dead. Here's the specs:

Motor: 350
Temp: 190-200
oil: 20-60 PSI (the warmer the engine the lower the pressure)
Charging amps: 15

What type of ignition? Original, HEI, MSD.......

Hei, Accel

Timing is set at 8 degrees TDC

Hei, Accel, 8 Dtdc

Check the ground for the battery. I had all manner of trouble with that. Lots of times I had to get a jump start. After the new engine and starter I didn't have the problem. I also have HEI now. I can't point to exactly what fixed it but the grounds were all redone, and a used a new high torque starter.

I checked all the grounds. Sounds like it may need a new starter.

Are you running headers? The starter can get hot and not crank until it cools. If that is the case, get a mini high torque starter and you problem will be solved. Just a possibility. Back "in the day" the problem you are having was a common problem especially with bbc.

I have headers so I installed a heat shield with the asbestoes backing. Starter has never not spun.

Make sure you have the heaviest gauge battery cables you can find. I had this same problem and swapped out my old cables with some 2 gauge wires and problem solved.