any 7/8" bars will work and with those big honkin' stock buckhorns, unless you go with 16" apes, you should have plenty of cable/wire/hose to work with - in fact you might have too much. But, I'm guessing the moto guzzi bars are more like cafe bars, so they should work fine.
I've got 5" straight risers and drag bars on my vt800 (nearly identical to your 86') and I'll likely get new cables this season since there's quite a bit of slack. I made the bar change because I also wanted less pullback - I'm 6' 2" as well.
The fender is molded to fit the taillight - so you can take off the light, but unless you can cover it up (maybe with the new seat?), there will be a big ugly space where the light used to be. If you do take it off, I think you'd have to replace it with another light - I'm not sure where you could re-position the stock light and have it look good. I'd vote for a side mount plate and light. When I put on my sidemount, I considered doing a new fender and putting the brake light on the mount, but in the end, I decided I liked the light where it was.