Until your vacuum stabilizes we won't know if the 6.5 will work or not, and the vacuum won't come up until the mixture leans (catch 22). Luckily, Power Valves are not real expensive, I would buy a couple different sizes, 8.5 to put in it now and a 6.5 incase it needs it later. Stick with the standard flow rates for now, with your altitude I doubt you’ll need higher flow rates.
8.5 & 6.5 are linked below;

https://www.holley.com/products/fuel...s/parts/125-85
https://www.holley.com/products/fuel...s/parts/125-65

With your altitude 2 Jet sizes smaller may not be enough, I would buy an assortment of #52, 54, 56, 58, 60, 62, 64;

https://www.holley.com/products/fuel...s/parts/230052

Tuning for this altitude is going to take some time and expense, be prepared to spend some bucks. Buy several sets of plugs, it'll probably foul a couple sets before we get it to run decent enough to dial it in.

Change the Power Valve and start with #64 primary jets, don't mess with the secondary jets until you get idle and cruise under control. once you get it running you can start reading the plugs.

No sense in looking at the plugs now, we know it's pig rich and they'll be black and probably wet.