MG Car Repair: 1970 MGB proper cable connection to 12v battery, alternator diodes, starter solenoid

Question
Put new battery in I believe I reversed the connection The red cable is going to the pos pole and the black cable is going to the neg pole on the battery  The black clamp is slightly larger and looks like it should be on the positive pole. The car is us neg. earth US model. The lights work and the starter but the car will not start. The dash lite is bright yellow. I am afraid to reverse the connections is that ok and will not fry the alternator diodes or wiring?   The car is com[letely restored and electrical system is in proper condition. please advise if the cables are proper or need to be reversed.

Answer
Hi Paul,
Your car originally came with two 6 volt batteries connected in series as such, One battery was connected (-) ground and the (+) was connected to a short jumper cable to the other battery (-) post and the (+) post of the second battery was connected to the long cable that ran up to the starter solenoid.
Many of these cars were converted to a single 12v battery. It is easy to ID an MG as to polarity on the cars with alternators. ALL MG CARS MUST BE NEGATIVE GROUND THAT HAVE AN ALTERNATOR.

Earlier MGs with a generator were (+) ground or (-)ground on later cars BUT!!! they can and some were converted to negative ground by polarizing the generator to a negative ground system. It was easy to do and the reason some did it was to use later high tech radios which were all negative ground.
Howard