Im looking at doing the same thing with an 800. Just going to use the 14 tank and short seat. Moving my battery to gain storage in seat! Works out cheaper if you want a different seat also.
I would call bdx and ask. They told me last year their tank could be used on either sled, m8 or 1100t. I know the C3 tank works on either one cuz I've used them. Just have to relocate the battery.
Finally got around to getting in touch with BDX and BCIL is correct, the tank will not fit on the 1100 regardless of what you do with the battery. Not that I doubted you BCIL, but I was sure hoping you were wrong.
So now that makes me wonder what is different and will the larger '14 tank on the 800 fit. I want more fuel capacity and I hate having gas cans strapped to my tunnel.
I Talked to Trail Tank at Haydays who I believe makes the one BDX uses on there 800's? They said the 800 model hits the throttle bodies on the 1100 but once they got back to there shop they were going to remould a tank to fit the 1100. So they are working on one. They said it would be available by the time snow flies but a call to them would help narrow it down.
We are, in fact, about to start making our new tank that will fit the 1100T. It will be about 13.5 gallons, save 4-5 pounds weight. Available in Black or clear. Price about $360 plus shipping. Should be shipping within 2 weeks.