Engine overhaul break in, ring seating

Maybe this post will grow some legs and help some others with my dilemma. I recently overhauled a John Deere 8400 tractor (~225 hp). Whole works. The rings were already on the pistons and in the sleeves and ready to drop in (I know cause I pulled one out and broke a ring trying to reassemble). I pulled the remaining pistons out just enough to get the wrist pin out and back in instead of risking another $190 ring set. I don't have or know of a dynamometer big enough to load a machine of that output, so I ran it just enough to check for leaks and readiness, then had the owner hitch up a heavy disc and pour the coal to it. The air filter was clean. It now has 12 hours on the overhaul and is leaking some oil out of the exhaust manifold to head gaskets. The rings are low tension in Rotella 15W 40. On a machine like this, or a big four wheel drive without a PTO, or machines which can't be continuously loaded (the list is endless), what are good ways to break in an engine, specifically the rings? The kits come with instructions on increasing load percentages and duration, but that is impossible to do without a controlled load bank. I've got a county tractor I overhauled that is doing the same thing. I plan to try to run in with a subsoiler and see if it clears up.

Engine overhaul break in, ring seating