I'm having trouble determing whether I need a 3" or a 4" closet bend and flange for this toilet. The low wall mount tank is marked KOHLER USA and both the tank and lid have Kohler part numbers marked K-6802. They were cast in early 1930. The bowl, which I assume was original along with the tank is marked 13/C (thirteen with a C below it) and was cast in late 1929. Most vintage toilets that I have seen online from this era are specified to have a 4" flange and closet bend. Some are even noted that a 4" x 3" closet bend will not work. I snapped an photo of the horn and it looks pretty small. What size should I use? (I have to cross a floor joist which makes 4" tough in a 2x10--steel would be involved--otherwise I'd just use a 4" drain and call it a day).
I'm also not sure of the rough in. When I re-installed the original flush ell, I measured 14-5/8" from the back of the tank to the centerline of the horn. Can I just shorten it up by 3/4" or so and install it on a 14" rough in? I hate to plumb up a non-standard rough-in and have problems in the future if the toilet is changed.