I just looked at my pump on Summit
and apparently the inlet is 3/8 and outlet is 5/16, so that fixes the pump to miata filter problem.
With your existing parts, I do not see how your going to adapt pushloc hose to your low pressure pre-filter fitting. Pushloc and low or high pressure fuel line are not interchangeable. Even if you manage to get the hose onto the fitting, pushloc is still far too rigid for hose clamps to be effective.
I looked on Jegs and Summit and couldn't find anything to do what you are describing. The bulkhead fitting in the tank is not available as a reducer.
This junction will not be under any pressure other than gravity, do you really think it will leak?
I would replace the tank fitting with a reducer. The tank may have pipe or a flared tube seat. Use an elbow with a hose fitting on one end, like an AN842. Use no more than 12 inches of low pressure fuel line to connect the pre-filter. Use a 3 inch piece to connect the pre-filter to the pump, replace the pump outlet fitting with a 5/16” hose fitting,
I'm not sure the pump outlet fitting is not interchangable/servicable. I also looked high and low for a 3/8" to 5/16" nipple to nipple adapter but came up empty. What now?
use a short piece of high pressure fuel hose to connect the miata filter, then another short piece of high pressure hose over the end of new hard “brake” line. Carefully cut the fitting off the tube with a die grinder and no reflaring will be required. Cut the tube off the fuel rail end of the hard line also.
I'm having a little trouble following you here.
Two inverted flare, high pressure fuel filters for another application with inverted flare fittings on the pump would be even better, using short hard lines. A high pressure filter works fine for low pressure applications.
Again, I don't think I can't change the fittings on the pump.
So you said I can't clamp the push lock hose down tightly, can I stretch 5/16" high pressure fuel line over a AN6 or 3/8" fitting?