The biggest problem with aftermarket efi is keeping fuel in the lines and air out. Every in-tank pump out there has some method of keeping the pickup submerged - even if only for a a short period while you turn or stop. This sump will be uncovered at some point when you slow down or stop, even if the tank is close to full.

Anyhow, why take the chance. If you're going to weld something into the tank, why not weld in the tank recess and install an in-tank pump. As for access, I suppose if you're so inclined you could make an access from the trunk but in reality, how likely is there to be a pump problem and even if there is a problem, Murphy's Law dictates that it will be in the middle of nowhere on a rainy Sunday night and you won't be able to do squat about it anyway.

What would you do if you high pressure "external" pump failed in the middle of nowhere, you're still stuck !!