No problem, I've posted this before. Here goes again. Yes, you can mount a 4 gal tank to the street model. Not really all that much work, just dollars to spend on an "R" seat, new white plastic and some bar risers.
Here is my '95 C with the Acerbis green tank. I can tell lots of stories about setting this thing up. Bottom line it works well without any serious issues.
4 gal Acerbis tank
"R" model seat rebuilt by Boejangles
FCR 39mm carb
Fatbars with risers
578cc big bore Vulcan piston upgrade
43mm WP/KTM front end
320mm EBC rotor
Happy Trails rack and boxes
It's all good! Lot's of work, yes, but I do love this bike. Cruises at 85 with a full load. Holds it's own with LC4's on the trail.