If the tube you have now are in good shape/have clean chrome than there's no reason to pay for -2 tubes- you can lower your forks with a pvc-pipe spacer (start a new thread for a good explanation on how to do it). If the reason you want to get the -2 cyclex tubes is because the one's you've got are in poor shape, I can sell you a perfect-chrome CB550 forkset for less than the new cyclex tubes (the 550 forks are 2" shorter than 750 and have the same lowers).
As for in the rear, how high it's gonna sit is gonna depend on the shock-length, and whether the lower shock mounts are in their normal place, or pushed back 4" with the swingarm-stretch.
How low you can go is gonna be determined by the exhaust you run- if it's up high/level with the frame-rails you'll prob be fine; if it's underneath/low to the ground you're gonna smack, scrape and grind it.
I've got an abandoned project/one of my donor bikes sittin on the aforementioned 550 forks and a 2-1/2" swingarm (from a CB650). It's got 12" shocks and sits real nice- I've not had it on the road but it works perfect in my imagination.image

http://s221.photobucket.com/albums/dd278/Chalfa1980/?action=view&current=DSCN1589.jpg