""No return line on the carb. I'm starting to think I might need to install one."" - With todays gas, this is almost a necessity with a street car.
""At the 15 minute mark, the fuel pressure dropped fast to about 3 lbs. The engine started to stumble. The fuel filter was almost dry."" - It sure seems to be a fuel delivery issue. The existing pump may be big enough but maybe it has an issue.
""The car does not have a fan shroud."" - Adding the shroud will help keep it cool once you get the fuel issue figured out.
If your headers are glowing red, and you have a fuel starvation/vapor lock issue, then the carb tuning at this time will not fix it. Concentrate on the fuel lines, fuel pump, fuel vent as mentioned.
------------------------------------- ""I bought an AEM wideband guage kit last winter. I also had bungs welded into each collector. Maybe Sunday I'll install the guage and wire it."" - You will find that a wideband kit is an excellent tuning tool and carb-tuning learning tool. Once you have this installed, the tuning process should go smoother for you. You could then give us A/F numbers and since we are all on the other side of the keyboards from you, we can give you better suggestions on where to start if this is a carb problem during your tuning process.
""I see listings for Standard flow, High flow and 4 window."" - What carb are you running? I would suggest using the style of PV that the carb manufacturer suggests. At 2600rpms, you should have enough vacuum to keep the PV closed.
