This one is a little hard to get exact, and because your setup is custom, you may have to do some pitch adjusting to get perfect.
More or less thinking out loud: So you are pretty much at a RXT255 level, plus intake, intercooler and tune. You basically lose 10% of current performance level for every 3,000 ft altitude (ie; 90% @ 3000, 81% @ 6000, 73% @ 9000, etc).
I’d say your best starting point is the 14/19 and you’ll pitch bend according to your target RPM range. This is a little less pitch (higher RPM), so you’ll likely want to adjust closer to 15/20. If you are keeping RPMs near the stock range, the 15/21 is a repitch from Riva that will be closer.