350 motor slow crank

So I just bought a '76 Cutlass Supreme with a rebuilt 350. Paid 1000
bucks for it not running. Get what you pay for right? So we figured
out that there was no fuel pressure. Thought I had to get a new fuel
pump but my friend and I discovered a kink in the hose from the fuel
line to the fuel pump, the hose was too short and wrapped to tight
making it kink. Replaced with a longer hose with new clamps. Started
right up beautifully. We adjusted the idle and got it to idle
perfectly.

I drove it about 1 mile turned it off for 10 minutes or so and jumped
back. Drove it less that 2 minutes up the street got out. Washed it
for about 30-45 minutes. Get back in to start it up and it turns over,
catches, and then dies immediately. Initially thought I flooded it, so
I let it sit for at least an hour. More than enought time I thought.

Lights all work inside and out. Sounds like the motor is cranking
sluggishly, leading me to believe the battery lost its charge. Here's
the thing though, the battery had only been driven on once.

I have no ideas. Is the alternator not doing its job? The starter is
working but it feels like it's cranking slowly.