Rebuilt Engine won't turn over

Got my rebuilt SD220 back. I'm ready to get this binder on the road. The starter turns the engine fine without the plugs in, but with the plugs in it won't turn. I have grounds from the battery to block. Same place on block to frame. Different place on block to firewall. I've had the battery load tested by two different places and one said the low reading was 6 and the other said 5.5. Not sure why they got different readings but they both said that's good. New 0g cables. Timing should be close. I did pull start it last weekend and it sat and idled for awhile but got hot so I shut it down. In the heat of the moment I forgot to top off the radiator once it started circulating so I'm hoping that the issue there. This battery/rebuilt starter did turn the engine over enough to start it before the rebuild.

So now I'm looking at voltage drop at the starter. After turning the key a few times this morning the battery was reading 6.30. Usually it reads 6.35-6.37 when charged. I put the positive lead of the voltmeter on the solenoid and the negative I attached to the starter case. It was still reading 6.30. With plugs out the voltage dropped to 5.05 and hung there while it easily turned over. I put the plugs back in, hit the key, engine wouldn't turn and it hung at 3.70. Did I perform the test correctly? I know 3.70 is bad but with the engine not even turning would it be normal for it to drop that low? Is 5.05 good with a free spinning engine with no plugs in? What additional voltage drop testing should I do if any?

When it comes to solving mysteries, I’m more of a hands on detective. My truck has long ago been “modernized”, but I seem to recall that when it was “original”, the positive side of the 6v battery went to ground and the negative side went to the starter. I’m sure CB89 will have the answer to this issue.

Fully charged lead acid battery is 2.2 volts per cell. So I would expect to see 6.6 volts not the 6.3 you saw.
If they used a 12 volt battery tester on the 6 volt for the test, it may not put enough load on it to give a valid test.
How tight is the engine to turn by hand? Plugs out it should turn easy by the fan blade, plugs in it should still turn but have some drag.
If you can't turn it easy by hand with the plugs out and not at all with the plugs in, then something is assembled too tight, and could be why it got hot when you pull started it.
If it isn't tight then I would suspect the battery or the starter.
A starter with worn or dry bushings will struggle. Battery is the the cheapest thing to try.
Positive or negative ground will make no difference to how it cranks. It will effect the charging system, so if put back different than it was, you'll need to polarize the generator.

I agree I ran the positive to the lower starter bolt, made it start easy, also I used a 1100 CCA 6 volt battery after I had the regulator / genny juices up.

I'm suspecting a tight engine. I thought that at first until it turned easy by the starter with the plugs out. The fan will not turn it by hand and even the 3/4" breaker bar on crank bolt takes some effort. The builder is closed today so I'm going to call him Monday. What are the likely culprits to make it tight? I'm assuming rod and main bearings would be the first/easiest thing to check. Rear main seal? Hopefully not the pistons.

If you had the motor assembled by the builder, I would have him come and take a look at what's going on. Something is to tight.
My 2 cents.

You mention 0g cables (single aught?) With a 6 volt system 00 awg (double aught) would be better for moving more current.

The rings in my BD 308 were real tight , the builder told me when I picked it up . I spun the motor without the plugs in and I think light oil in the cylinder with the valve cover off untill I could see oil in the top end , I think I even had the front hollow bolt on the valve train assembly out so I could see oil there . I enlarged the battery box so I could fit the biggest battery with the most amps , anyway I cranked it alot with the plugs out before I tried to start it . I also ran the positive ground from the battery to the starter bolt in a thick cable , as recomended from this site . Start up oil is also a good Idea has a lot of zinc in it , good for these old motors
I use Joe Gibbs racing motor oil in it still , have around 5000 miles on it .
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dumb question, but did you put oil in it? I knew a guy in the 70s who had a shop rebuild his GRD motor, they assembled it put it in the truck,
called him to get it, they didn't put oil in, he didn't check,
he started it up and drove it home, fried the crank, melted ring lands in the pistons.

Hardly a dumb question, bedrock Jon. I did not see anyone mention checking the oil.