That's an interesting thought that the 750/900 swing arm might fit a 1978 750F2, but the Metmachex website dosn't include the older machine. I would suggest asking them directly if the 750F2 & the 900FZ are the same, but I wouldn't hold my breath.
When I looked for a wider rear wheel, I found that I could go up to a 4.25" x18" rim specially made for me by CMA Wheels up in Suffolk/Norfolk. With this wheel I fitted a 160/70x18 Metzeler MEZ1 CompK, but only after making a custom torque arm for the rear caliper & chopping out a portion of the chain guard.
Unfortunately the guy is no longer alive, from what i've heard, & i'm not sure who makes specials to order now that Dymag no longer make them.