Newly assembled Flatty cranking force??

[Kilroy]

Help me just validate my assumption here...

I've got an assembled flathead that hasn't been run yet. I can turn it over with a breaker bar (plugs in), and cranked it with the plugs out with the starter to make sure the oil was flowing, etc...
But with the plugs in, the starter don't move it and the wires start smoking. I've tried both 6v starters.

So I'm assuming it's just the starters are tired and I've killed them both. Sound logical?

There's no chance the motor is too tight for the stock starter, if I can crank it by hand (breaker bar), right?

It's a Merc with Navarro standard heads, and I didn't relieve it. So the compression is high, but not crazy.