318, mild 340ish cam, Holley 600 which was professionally rebuilt. Upon reinstall, motor starts OK, can't find any vacuum leaks. 5 minutes in, motor refuses to run unless I maintain a very high idle with lots of manual choke. I adjusted idle mixture and confirmed bowl levels with no improvement. My suspicion is too low a power valve, a new 6.5 is installed. I don't have a vac guage. Do you think I'm on the right track to suspect the PV value?

Vac leak or idle circuit issue (too lean). EDIT your initial is in the ballpark?

Yessir. Initial 10 deg. Lean...come to think of it, my exhaust doesn't smell at all gassy.

you might bump the initial up to 20 & see how it acts. I was running 18 on a bone stock 318

Like Robert said, try 20 initial instead of 10. Timing first, then carb adjustments.

""I don't have a vac guage""
- Try and pick up a hand held vacuum gage if you do not want one mounted in the car. The vacuum gage will help you tune the carb. You are looking for the highest "solid" vacuum reading that you can achieve with your carb. The hand held unit can be connected or disconnected at your convenience, but you need one to help tune the carb. The vacuum gage will also help you setup your PV size once you get the idle circuit straightened out.

""Upon reinstall, motor starts OK, can't find any vacuum leaks. 5 minutes in, motor refuses to run unless I maintain a very high idle with lots of manual choke.""
- It sounds like your choke is setup good but when the choke opens up, the idle circuit is too lean to maintain idle.

Thanks! Next tune session may be a few days away.