Timing curve for my big-cam 451?

I've been chewing this over for a while... did some more reading (including my thread in the Elect/Ignition forum from Feb. 2020). Since I have a high idle, I need to "cut off the bottom" (as Mattax has said) of the advance curve. That means not using the FBO plate, but instead welding the inside of the slots.

I will always need the heavy spring with loop which will provide high-rpm additional advance to offset the inherent retard in the electronic ignition and the rev limiter/nitrous timing retard unit. It appears that the OEM heavy loop spring allows 14 deg before it engages.

Tell me if this plan sounds reasonable:

Initial will be 24 deg, can't use more for hot starting.
Desired all-in is 36 deg.

Heavy spring engages at 38 (14 deg from initial to loop engagement) by which time the electronics are already starting to retard a degree or two, thus staying at 36.

Slots need to allow at least 42 deg. for ~6500 rpm shifts (it won't actually get that high due to the minimum 1 deg/1000 rpm retard in the electronics). So that is 42-24 = 18 crank degrees, or 9 dist.

I've already ordered an 18/9 tower from Halifax Hops and it should be here tomorrow... but now I'm wondering if I should take the otherwise useless stock 26/13 piece and weld up the inside of its slots to 18/9 (.390")?

The only remaining piece of the puzzle (besides the vac advance, which I'll worry about once the mech timing is right) is finding a suitable spring that will be all-in at approximately 3000 rpm. I think the OEM blue spring may be close... thoughts? :)