Dual springs are better in every way but price.
Stock pushrod length should fit. Your likely going to find out what a bent "stock" pushrod looks like. They weren't designed for. 540 lift with a 340lb spring.
The stock electronic ignition is fine if its in good shape. We run several original chrysler distributors and just use it as a trigger for a msd 6al. No issue to 8500 rpm. The timing does flutter a tiny bit.
Back when we had a turbocharged 318 in our 72 satellite it had orange box and stock distributer. It never missed a lick while doing a burn out with the stock 7.5k+ tack pegged in drive

. Not sure how many rpm it was running, but the car had 3.23 and the 150 mph speedometer was past the odometer and bouncing against the peg. It never missed once for what seemed like a eternity. I let up when the tires came apart about 400 ft later. Black marks were so dark they remained till they redid the highway....the good old days when we didn't have all this traffic and cell phones.