I went with Kevin's recommendation to use Joe Gibbs BR1, which is a 15W-50 break in oil from the start. After a few hundred miles I switched to Joe Gibbs HR1, also a 15W-50 oil.
I spent quite a bit of time today reading up on oil viscosity. Bobistheoilguy has a "viscosity 101" through 201 and this explains viscosity vs. oil flow rate in generic terms. All xW-50 oils will have the same viscosity at operating temperature. My oil pump is going to flow oil at a rate increasing (approximately) linear with RPM until the pressure exceeds the bypass valve setting. After that point it will flow at a constant rate.
My engine oil should operate at about 210 degrees F. A race motor putting out max power will operate substantially higher, around 300 degrees, thus requiring a higher viscosity number to survive.
Melling doesn't advertise what the pump bypass pressure is, but I have had a mechanical gauge on this engine for initial startup and found this to be 80psi. Using a 50 weight oil, 80psi pressure will be exceeded at relatively low RPM.
Bob has some interesting charts in this article here: http://www.bobistheoilguy.com/motor-oil-109/
Bob's charts for 30 and 40 weight oil and a high volume pump suggests that 80 psi is reached with a 50 weight oil at less than 1500 RPM. After that, there will be no increase in flow. Oil flow rate is what lubricates plain bearings, so the bearings aren't being lubricated any better at 5500 RPM than at 1500 RPM.
From this I could take a (poorly) educated guess, that a 30 weight oil or possibly lower is going to maximize oil flow volume for my street motor that at 3500 RPM and will rarely see higher engine speeds.
Bob recommends experimenting with lower viscosity oils step-wise. For my application, that means switching from 50 to 40 weight.
Based on this I don't see a problem at all going to a conventional 10W-40 oil, approved by Howards for their lifters, and using a mechanical gauge to verify oil pressures and then possibly going a step further to 30 weight.