Fin:
My TN65 has 16.9x28 and with the rear tires set at the minimum width I am ~74" from the outside edge of the tread. That dimension does not include the buldge on the sidewalls which is probably another inch per side. The 14.9's are a lot smaller than the 16.9, but I am still not sure you will make it if your opening is exactly 72". If you plan on using a FEL then you may be better off widening the opening rather than making a compromise on the tires. In my experience if you plan on running this tractor on any grass/lawn that you don't want torn up you will want to get the 16.9 x30's since they get the 11.2x24's up front. With the 16.9x28's mine has the 9.5x24 which are significantly smaller in footprint than the 11.2's. If I had it to do over, I would have waited for the larger tires. We have both a TN65 and a 4630. The 4630 is heavier than the TN and has the 16.9x30/11.2x24 combination and does not damage the yard at all in comparison to the TN with the smaller tires.
Our TN65 has the 8x8 shuttle trans and it is plenty slow for mowing. This may not apply to the more powerfull TN75, but I would go with the 12x12 if I were doing it over. The problem that we run into with the 8x8 is that you really only use 2-3-4-5 the rest are either too slow or too fast/not enough power. For loader work I find myself digging with the 2-3 gears (mostly 2) and for plowing/discing/cultimulching/
haybining/baling/raking/etc. I find that we are usually in 4-5, sometimes 3 if it is hilly. I would imagine that the 12x12 would give you a couple more useable gears within (in between) these ranges. The 4630 has a 16x16 trans and is easier to find a more comfortable/workable speed.
Good luck with your purchase, I hope you enjoy your TN as much as I do.
Jim