The pressure relief valve is situated just before the existing filter housing so to take oil before this would be at maximum pump pressure, it follows therefore that it is best to take the output feed after the existing filter housing, into the new full flow filter head then to the engine block feed position. You are best to remove any filter/strainers in the exising filter housing as this only causes a pressure drop before the new filter which is to be avoided.
Your pressure gauge should be connected to the output side of the new filter so you know the actual pressure entering the engine, it may be necessary to adjust the pressure relief valve based upon the reading you get from the new position.
Unfortunately the pressure relief valve should ideally be positioned after the filter as there is always a pressure drop going through the filters, the less obstruction the better. One can also have pressure drops through the pipework which is not very large on AC engines.
The key is ensuring good oil flow at sensible pressures & taking pressure measurements as close to the input to the bearings as possable.
By the way there are several differances in the range of sumps produced for the engine over the years!
I hope this assists.
Robin