427 Contemporary Cobra - Charging Issue

[TheBDP]

First-time post here...

'66 427 Side Oiler Contemporary Classic (CCX34026) - Registered as a 99

I can't seem to find the root cause of a charging issue, and looking for pointers on where I might want to start.

The original alternator was bad from purchase(tested at a local Auto-Zone) -
I purchased a new Powermaster 150 AMP, and swapped it out.

Gauges now show it charging. But when I throw the battery tender on, it's showing a 60% capacity after about a 30-minute drive. Drops to about 30% after an hour's drive. I'm worried it'll die on the road.

No documentation is available from the original build, so looking for any pointers on where I should start chasing this gremlin at. Fuses look fine, and I have no idea if this car has ever had a fully charging system or not.

I'm pretty mechanical, and good with my hands, but I'm not too proud to admit when I don't know something. Any help would be greatly appreciated. Trying to avoid taking it to a pro, if it's something reasonably straightforward.

I don't have access to a lift, which seems to be my most consistent issue with the car. I can't seem to get under it to access everything.