dodge ram magnum 360 no spark

The slow crank can only be a few things....weak battery, bad cables, ground, or connections, failing starter, or binding engine.....is this a rebuilt engine or good used that you heard run? If it's the same starter, cables, and battery that you had on the old engine, and they were working fine then, I would say that they are probably not the problem. That leaves ground, connections, and something wrong with the engine.
As for no spark, the crank sensor doesn't need a good ground to the block. It's just a loop that picks up a magnetic pulse and transmits that to the computer. If you overheated some wires with your ground problem, you may have damaged the harness, computer, or other system component. You may need to get a FSM and do some trouble shooting. If it were me, I'd get the correct computer for the 360. They made them different for a reason. Doesn't hurt to have a spare when you're east of nowhere when it fails either.