Decision: 5x4 (SBP) or 5x4.5?

Actually. A large percentage of old mopars road racing use factory braking systems.
It all depends on the sanctioning body, and with vintage racing groups the emphasis is running like it was back in the day.
And for autocross, the factory systems are more than adequate.
For track days, then you are free to use what you want. I'm not going to criticise anyone from using a lighter sytem or a larger rotor when there is a reason for it.
The O.P. said he wanted front power disks, but not for any specific competition or track use. So that's what I was trying to keep my comments to. Any of these options will work, and work pretty well if everything is matched up.

Heh. just noticed in mopax archive that there was some difficulty sourcing 11.75 x 1 rotors in 2014..
These things go in cycles. I wouldn't sweat it.