I'd suggest removing all the belts while diagnosing the slow cranking problem, and once you have determined whether or not the engine has an internal problem start adding belt driven accessories back on one at a time. The recent rebuild concerns me- - - -it's possible some sort of mistake was made that is just now coming to be noticed. I've seen situations where a less than clean assembly allowed debris to accumulate in a rod or main bearing and result in the bearing and crankshaft clearance tightening up. That happened on a competitor's racing engine once, and he ended up losing a $10K rotating assembly over it.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!