(quoted from post at 16:17:33 01/01/19)
(quoted from post at 13:42:27 01/01/19) Do not use a reliance kit. The pistons are made wrong. The reliefs are too deep and your 20% + low on compression right off the bat. Gaskets are not very good either. They will not admit they have a problem. It's a big deal if you want it to cold start. The Arrow sleeve and piston kit is the best by far. More expensive but very worth it. I get them through Maibach tractor. Done right these engines will start un assisted below zero. Be very pickey and spare nothing when it comes to a 310 or you will pay twice later. Arrow Pistons , single piece pin bushings, agco gaskets, arp rod bolts flange head ferry bolts for head bolts and new valve seats set to the top end of the spec. Also make absolutely sure the sleeve protrusion is correct. I also send my harmonic balancers out to be rebuilt. Maibach was working on a improved head gasket. I haven't heard how that's working out yet.
Low oil pressure is probably caused by worn rod/main/cam bearings and/or worn oil pump.
You say it started and ran good.
The 1855 I am rebuilding started and ran good too. I never figured the injectors where gonna be that bad when I pop tested them, all but one had a opening pressure under 1500 PSI and all had a bad spray pattern and none chattered on opening.5 outa the 6 had broken springs in them.
If the pump seals are leaking then you're better off to rebuild that pump as well while you're at it.
My engine had 40 PSI oil pressure at idle, yet upon teardown it showed the crank needed a grind.