95% of electric fuel pumps are not made to "pull" fuel from the tank. Some can do it with decent reliability, (myself included) if they are close to the tank, but it is not recommended by most.
I'm surprised it is even getting fuel period where you have the pump mounted! That is a very long run with a huge uphill before the pump even sees fuel. Mount that sucker under the cab as close as you can get to the tank and it will be much happier.
Even better would be to find someone to weld a fitting to the bottom of your tank so the fuel pump doesn't have to pull fuel uphill at all.
I wouldn't change the fuel line size until trying to relocate the pump. That little fuel line can support more power than most people think.
Sweet build by the way! I always wondered how well a turbo'd straight 6 chevy would tow
