USPSA minimum power factor is 125, not 120. I do not believe it is possible to load 9x18 up to a 125 power factor, and remember the steel poppers are calibrated to fall with that power. Minimum cartridge is: ".38 cal. / 9x19 mm (0.354” x 0.748”)" so 9x18 is not quite enough.
As for using a single stack, it is USPSA - In
Heavy Metal, there are no race guns; you can use 10 round mags (HM is a Limited 10 handgun, not Single Stack), and that is easier to run than 8 rds. If they say you are limited to 8, either they are not aware of the latest rules, or it is not USPSA.
Ask the match director what the rifle course looks like. It might be M1 friendly, might not be. Either way, know your zeroes. M1 shooters are rare, although every once in a while someone throws down a "WWII Challenge" and a few show up, basically Heavy Metal Limited with an M1.
Lee