I keep a 1/8 NPT tap on me.
If the head has a port on the side, I usually use this as my oil feed. if I have to go into the back of the block, I take the oil filter off, have a contraption of hoses to hook to the oil out (heading to the oem sender) and remove the oem sender. I plumb 10PSI regulated to the oil port as to keep derbis from going into the oil system. Then just get the good ol tap and tap her out to 1/8 NPT... doesn't take long.
This is best done with then engine disassembled; Every time I build a motor I tap out the NPST ports to NPT. This makes life less complicated when installing a turbo kit.
Using a T off the block is fine as long as it clears everything and doesnt have a chance of hitting something or something hitting it and snapping the threads off in the block.
This is the Z6 port location I use.
On a P72 head you can see the OEM port plug at the lower left of the head. Just above the engine stand bolt.
Whichever port you use; just use a port if available with 10PSI to keep shavings out of the motor while tapping the threads.
If you try and cram a 1/8NPT fitting into the OEM 1/8NPST, you will only get 2-3 threads of contact; the NPST is a more tapered thread then an NPT.