After checking those grounds and checking all the positive cable connections and being sure the cables are BIG.
WAIT WAIT WAIT for Jerry to address your "rebuild" as to serious binding/new rings and bearings and tolerances/first start procedure oiling/bad rebuild/etc.
Just to be safe.
Meanwhile explain what you have done or were told to do before first start. Spin oil pump? Turn over without plugs? Special oil? RPM's and length of first run. Anything?
You only need one coil. Who the heck is telling you these things?Be aware: If starter "bench test" was under load, that's one thing, but if just spun on the bench, it can fail under load of turning motor over after install.
If battery is not load tested , it can show good volts at rest, but break down under starter load. So can cables and their clamps and grounds. Don't be fooled by the word "new" or "rebuilt" or "bench tested". Motor is new rebuild, starter is bench test like new, battery is new, cables are new, coil is new....so guess what, something new/rebuilt/tested is bad.
Here are some other items, not covered, that may possibly cause slow cranking:
1. Foot start button switch
2. Solenoid
3. Ignition switch
4. The wiring for 1, 2, 3
5. Timing WAY off
6. Stuck heat riser
7. Water in your oil, check the dip stick
8. Anything funky you forgot to tell us
