The single shafts are a PITA to upgrade.
IIRC OD is 7/8"?
The OD is far the biggest factor, but needs everything changed (stands, rockers).
Comparo, with 7/8" OD with 1/8" wall as 100%:
1" OD with 1/8" wall = +57.7%

The material doesn't matter at all - moly isn't less bendy than mild steel and alum is worthless.
Wall thickness is what's left, and it doesn't do much (although it's cheap and there's no down-side). You don't even really need it hardened or chromed if you have bushings (needle bearings or plain rocker ID: absolutely necessary).

Comparo, with 7/8" OD with 1/8" wall as 100%:
7/8" OD with 1/4" wall = +30.6%
7/8" OD with 3/8" wall = +35.1%
Give me your wall thcikness and I'll do the math for you.

Another stand between the existing might do something, but not much room.
Upsizing all the stand hardware will improve location (i.e., 5/16" > 3/8", etc.) but the heads have to be tapped, not much room in the stuff, etc.
Extending all stand/cap hardware upward and using a fabricated tie-bar will do something, but the obvious problems:
1. must clear the rocker path, so tall
2. no clearance in the cover
3. must be reamed to accept stand studs