Almost sounds like vapor lock to me. Symptoms are consistent with vapor lock.
When you replumbed the truck, were you careful to follow the original fuel line paths, or did you re-route plumbing in any way? Is the fuel line close to the manifolds in any way or does it pass by any heat sources?
In summary, vapor lock is where the fuel heats and "boils" and then can not flow. Try to isolate the fuel lines and if you have the time and patience, try making up a "jumper" line and install it from the fuel pump up and over the top of the rocker and into the carb so that it is as far away from heat sources as possible. See if this cures the problem. If yes, then you know you need to re-route your plumbing (to the original paths?)
The new formulated unleaded fuels are much more prone to vapor lock than fuels were when your 248 was made. Add to that the high compression of the 248 and you have a formula for vapor lock. This is a problem in aircraft engines too and if you dig around the web, you will probably find quite a bit of information on it.
Also, there are fuel additives that provide some reduction of vapor lock potential, but my guess is that the restoration work fuel line path changed from the original configuration. High test fuels are more prone to vapor lock too. There's an old trick of adding a little diesel fuel to your gasoline to prevent vapor lock, but I strongly discourage it.
Vapor lock is my guess.
