My original plan was the same as yours, I did not want to remove the belly pan . . . then I "slipped off the slope". Before the "Slip", when I was following my plan, I found, because the outriggers are fairly light gage (and often times rusty/corroded), that the self-tapping screws easily "spun" and stripped out the threads they had just created. After I slipped I bought the elevator bolts and added them as well.
How much floor are you replacing? If it's pretty extensive I'd be concerned. If it's "limited" then maybe switch to hand tightening just before the screws bottom out. If you use the Vintage Trailer Supply versions (because of they're relatively large head) maybe even drill a small countersink seat so the screw head doesn't have to create it. The other thing I'd do is use more of them than you otherwise would have - maybe 6" on center.
Just my dos centavos. Good Luck and stay away from the edge of that slope.
Aluminumbskull with Led Balloon in Drag
Birch Plywood and Aluminum go together like
Peanut Butter and Chocolate