firstly the oil will drain back to the sump from the cylinder head, basicaly it goes through the rocker gear shaft, through the rockers and the excess will drain back into the sump via
the holes in the head for the pushrods onto the lifters & back into the sump.
The oil pressure should be 60 pounds at higher rpm, With the oil pressure firstly you will
need to look at the oil filter cartridge.
Does it mount upside down onto an alloy bracket that then bolts to the engine...?
If so, it will have an oil pressure releif valve in the alloy housing that should be checked.
When you removed the oil pressure relief valve & spring you said no oil pressure....?
this could mean that the spring has to much tension on it & not allowing the releif
valve to do its job. This valve is the same as an mgb valve.....
If you need any more help P.M me as mine is up and running no problems and will help where i can.