[Yosemite Sam]

ol'George may be exactly right, but you may also want to consider that your freshly rebuilt engine is still a little tight, it may just need some more run time to loosen it up.

Try hooking jumper cables from your car/truck directly to your S/G (+ to "A" terminal on S/G, Ground to tractor last hook up). If it spins very fast you can rule out compression relief and a weak S/G.

Then put jumper cables on tractor battery and use key to start. If it still spins over fast then your wiring is ok.

There are a number of variables and eliminating one thing at a time should be the easiest way get to the real problem.