What pushed me away from running in-tank pumps the last time I was setting up a tank was that I couldn't find any in-tank pumps that had threaded fittings on either end. Going in-tank seemed to mean clipping a sock on one end, and some sort of fuel hose on the other, while I wanted to run fittings and hardline - walbro pickups on one end, hardline out to fuel rail on other. Since I wasn't seeing what I needed I just went external and that was that.
I have another tank I need to plumb soon, and I'd like to go dual pumps in-tank, with hardline to hydramat on one end of the pump, and hardline out side of tank on the other. My main question is what is the main difference between an in-tank pump and an external pump? What happens if you run an in-tank pump outside the tank - other than it being a pain in the ass to attach to the inlet? What happens if you run an external pump submerged in fuel?
I talked to someone at Holley who had no idea not too long ago (surprise). Last time I was ordering fittings from G&J Aircraft I asked them and was told that they'd been running an A series Aeromotive external pump in-tank for years on one of their cars with no issues, and didn't have much to say other than that. I suppose the next call should be to Aeromotive directly, but I never got that far...
My last setup that I pussed out and went external - put a 1500 psi check valve in-line after each pump before it Y'd back together. If going in-pump, might be good to have checks and Y happen outside tank for easier diagnosis if pump not responding?
Not sure if any of that is helpful or if I'm just posting random thoughts. I will say though, for anyone needing to order fittings - I've found that if I draw out my plumbing schematics on paper with all types/sizes of fittings, I can just email that to G&J Aircraft and they will piece my order together
edit: Another random thought - another difficulty I found when wanting to plan out an in-tank pump setup, is it seems every mfg option is to have pumps and pickups mounted to the top plate. If you're running baffles in your tank, that means you have to smash pump/pickups into your baffles as you bolt in the top plate, which doesn't make any sense to me. What makes sense is to have your pumps mounted in the tank, pickup mounted in the tank, hardline connecting everything - all separate from top plate. Baffling can be packed in and top plate installed, and it's all separate and clean