From experience, don't fit a T piece for an oil pressure gauge, even more so if it is electric. I have mention elsewhere that, years ago I had one crack from the combined weight of the gauge sender unit & low pressure warning sender due to the vibration of the motor. It sprayed oil all over the engine bay and I couldn't drive any further. Not a road side repair trying to remove the snapped off thread from the block. Most T pieces are brass, they are not that strong and the quality of these fittings is terrible compared to 30 years ago when mine cracked.
You have a few choices
First and the most practical, get a hose made up buy a local hydraulic hose repairer so you can screw one end into the existing position on the motor and with a T at the other end mount it of the side of the engine bay so it supports the weight of the gauge sender and warning switch.
Second, if you chose to run an oil cooler, the sandwich plate that fits between the engine block and the oil filter usually has 1 or 2 additional holes for oil pressure and temp sensors.
Thirdly, which is what I chose to do on my conversion, I have used a special VDO sender unit that has does both functions. It has 2 terminals, one for the signal to the oil pressure gauge and the other for the signal wire to the low oil pressure warning light. You would probably have to use a VDO gauge too, not sure if there's a Smiths equivalent
Other may have some other ideas, but these have worked for me
Cheers Stephen