My 2 bobs worth.
5/16" sounds a bit small, volume and pressure are what every fuel driven car need.
I've always used 3/8" standard dayco braided line on everything I've built, even my LPG powered pulsejet, (pulled about 42 hp too
)
They were all small engines, most of the guys that had thier speedway cars at home on methanol ran 1/2" line minimum. I know it's a different fuel, but they were only 250 crossflow Falcons. 5/16" line is for mums car.
Other obvious suspects, vapourlock, but on a fuel injected engine a fuel return/ bleed to tank (however small) would eliminate this?
And, any chance your inlet outlets on the surge tank could be arse about face? I've only ever made that type of turnout for carby engines before.
There are some cluey ford lads at the fordmod forum site too, some of them are nearly as fanatical as cobraists.
Good luck