Looking at my drawings I see the oil coming from the pump to a common oil gallery. the relief valve then controls the max pressure in the engine. when excess pressure is sensed the valve opens and dumps the oil back to the base. The bypass filter takes oil from the oil gallery and returns the oil back to the base behind the relief valve. Increasing the spring tension will only have an effect on Max pressure. If you have low pressure you may have a faulty (leaking) relief valve, a stuck open relief valve, a weak oil pump, or a bearing problem furthur along the engine.
As for making a full flow system on the engine , I have read where modifications are made right at the oil pump. The base where the pump mounts is drilled and oil is diverted thru a full flow filter before entering the engine. On these engines I feel the by-pass is sufficient.