Will the stock specs leave power on the table?
It really depends on what the stock distributor has in it, and how much that cam changes things.
A lumpy cam likes more initial timing than a stock cam. Also lower compression will want a bit more advance high compression. You might end up setting as high as 18 degrees at idle but 15-16 seems like a good starting point. If there is enough vacuum at idle to get the vacuum advance to contribute at idle (in gear) then it may be fine with 12-15 inital.

For total advance (w/o vacuum) Mopar Performance recommended 38 deg BTDC for production iron head B/RB engines. With vacuum they recommended 56 degree BTDC for these engines. (Instruction Sheet DCF-194 for Mopar Performance Electronic Ignition Kit p.6)

Be conservative. Start with these numbers or less. Because under load (like top gear, WOT) too much advance can be destructive.

You could wait until the engine is done. Find what initial it likes. Then knowing how much advance is in the slots, weld up as needed to get the initial and total you want.

Which Springs?
Yes. the early performance guidance (eg. Master Tech bulletin 1969) advised removing the heavy spring and stretching the light spring. However, if you shorten the outside of the slots, the heavy spring will not come into play very much and could actually be useful - especially on a heavy car.

There are competing theories on the best approach. This early advice is not the only way, and even DC/DC used other approaches. For example later on DC/MP liked to use a lot of mechanical advance, a very light spring and a long looped heavy spring. The engines would start easily and get roughly the advance needed at idle. Full advance might be 800-1200 rpm, with heavy spring allowing only a degree or two from there to the top. The p-partr tach drive dizzy was set up that way. My opinion is that this makes most sense for a very high compression race car, but not optimum for lower compression mixed use cars.

My suggestion for your application is to first get the slot length. Then, if the heavy spring with long loop is only controlling only a degree or two at the top of the curve (2500-4000) leave it for now. Then try lighter springs on the other side to see if bringing the curve to full advance earlier is beneficial.

With a heavy car and stock convertor or manual trans, the shorter slots might be enough. In fact you may find that instead of 1 light spring and one delayed heavy spring, it prefers two light springs.