Advance spring

There are no spring kits with a variety of primary and secondary springs for Chrysler made distributors
There were spring kits for distributors with Mallory YH and YT advances. That's a total side track and will not go there.

It is not really a question of an entirely unknown engine. We know a little bit about it and it responded to the changes in a predictable way.

We don't know much about the distributors.
That is the big unknown or both distributors.

The original distributor, that seemed sluggish. IF it still has its ID plate on it, its probably unmessed with. We can use that part number and the shop manual for that year to know what the curves looked like. If the ID plate is missing, then we can still turn to the shop manual, but will need to verify the advance with a timing light and tach.

The new distributor, with its original springs, or its replacement springs - the only way to know the curves is to measure timing vs. rpm from as slow as it will run to as high an rpm as one feels comfortable.
^^^^^^^^^^^^^This the starting point. ^^^^^^^^^^^^^^^^^^
Knowing this info, the curve can be altered.
If you do both sets of springs, and the old distributor, then you'll really have some baselines to work from.
When changing springs, write down the distributor degree info stamped on the governor, and note if the slots are parallel or slanted.

The changes may or may not involve replacing springs.
Been through this a few times. A search should turn up several posts explaining how to do it.

Yes it can be a lot of work just to measure the timing curves, and then more to make the adjustments.
That's where distributor machines come in. Yes that involves buying one or paying someone who bought one.
Alternatively one can buy a distributor that will be pretty close to begin with. Rick E claims to sell an exact copy of the Chrysler built MP distributors.
Halifaxhops sometimes has original Chrysler built MP distributors or, if you can wait, he can customize a distributor for your engine.