No spark and i can only get 5V at the coil.

You do know that if you are using a 1975 Factory Service Manual, as it appears you are, then it has procedures to test every single aspect of the ignition system. ECU, ballast, pick-up module in the distributor, ignition coil itself. All of it. That's what the techs originally used at the dealership.

Have you VERIFIED there is no spark at the plugs? That means remove one, lay it against a ground and then crank the engine over. You could have something as simple as a corroded terminal in the cap or a broken coil wire.

Accel coils are junk. You can get the correct replacement ignition coil for about $19 in stock at pretty much any Autozone on the planet. Ignore the drama about the "ECU might have a ballast in it blah blah". Whether it does or not Mopar spec'd what ballast and what coil to use so put it back stock.

I think i mentioned this in another thread you started but the ballast supplies two separate voltages at two separate times:

Cranking voltage which is supplied to the coil when the key is in START. That's full 12V to help the engine start faster.

Running voltage which is resisted down to about 6-7V at the coil. That's because the coil tends to overheat and fail at full voltage. That voltage is supplied when the key is in ON position.

That's why when the run side of the ballast fails, since it does all the work, the engine starts as long as the key is in START and sending that full 12v to the coil.

Here's a challenge:

Imagine it's 1979 and you are a newer line tech at FABO Chrysler-Plymouth, and you just had a 4yo '75 Dart come in with a no-start. It's assigned to you to repair. The customer needs their car back as soon as possible because it's their only car.

Every hour you spend working on that car not only charges the customer the hourly labor rate, but keeps you from taking on other jobs. Not only that, but the customer is on the hook for any replacement parts since that car is now out of warranty.

So, what do you do? Do you take guesses and try different parts, all of which are billed to the customer. Do you fart around wasting diagnostic time?

Or do you utilize the tools at hand, namely the proper electrical test equipment( a spark tester and an electrical meter) plus the FSM with it's detailed diagnostic instructions to quickly isolate the real problem, repair it and get the car back to the customer without excessive labor and part costs, which they would most certainly balk at paying?

:)