On the QEC-

Go to the fwd head/bank area-


Each side will have a bleeder plug (hex plug with saftey wire) on the O/B side just fwd of the exhaust-

On the mk 61 head this is the oil feed point, on the mk 58 this is a bleeder plug to check oil flow during pre-oiling.
The main oil feed point on a 58 is underneath the fwd head and there is a hole that the feed tube interconnects with when the head is dropped into position.We have 58 head/banks on the 61 bottom end. They are pretty much the same in how they fit, (61 vs 58), just slight differences in the oil feed and breather set ups.
Back to creating a pre oil set up for your Shackleton. I would engineer some fittings to connect a 1/4 to 3/8" feed line to each of these bleed points. T these points together with a check valve to the pressure side of the pre oil pump. The Spit runs about 12 psi on the pre oil system. You need a decent flow rate with the oil. I would keep pressure about 10-20 psi.
I really don't know what thread or size these are. The British system of doing things is hard for me to master.
If you can also figure out a way to pre oil the main bearings as well it will help the engine to last longer. You have to isolate the cam oiling pre oil system which is low pressure and the main oil system is high pressure so whether you use a manually selectable valve or another check valve to accomplish this.
But you can connect to this point at the front of the engine-

It feeds right into the oil galley that feeds the main bearings. I pre oiled the Spit at this point as this on a Mk 61 supplies oil pressure to the ATD. On the 58 you will have to drill and tap into the small oval plate with 2 nuts seen above in the photo.
Rich