First and foremost before you do anything else please insert a fuse holder inline with perhaps a 5 amp fuse/ maybe a 10 at most before you apply any other power. Test each and every circuit as it is built and connected that way you can work through the wiring harness in series of steps. Save the heavy load of a starter as the grand finale. Hopefully nothing too much got damaged.
Something in one of your circuits is wired backwards. It manifested itself in where the heaviest load was being applied.
BTW-Engine doesn't care if + or - ground. Personally I will rebuild a '59 BE I am acquiring(all wiring hardness is out of car and in a box) as negative ground. Just to make things easier. My '68 Sprite Bugsy(negative ground car) has a + Ground '66 1098 under the hood. DPO had been unable to get things to run in this car. Car would start and engine would run if you fed it gas but was not getting fuel from the tank. I got the car for a good price as he'd been beating his head against the wall trying to get it to run for 6 months. After cogitating on this one for a week or so I looked at the way car was connected. Battery had been connected as positive ground, light bulb came on and I unbolted the battery and turned it around. Turned key, heard click click from fuel pump, gave it a moment and turned the key and engine roared to life. I simply needed to repolarize the generator to get a + ground engine to work as - ground. Nothing smoked. Instruments worked either + or - ground.
So the net is you've got something miswired. Start small, put in a fuse to save the woring harness, and start testing in small steps.