I see absolutely no issue using the top suction line on a tank with any engine mounted fuel pump. Once there is fuel in the line the work on the pump is just the same as if it were draining out the bottom. Think of the pickup and first few feet of fuel line as a siphon, no work necessary to move fuel once primed. Those mechanical fuel pumps are dead simple and very reliable, it won't care in the least if you use a pickup tube or fitting on the bottom of the tank.
That said it sounds like the simplest solution is tap the metric tank fitting for 1/8" NPT and use the tank as if it were the original. Plug the extra suction line on top.
Grigg
