Each tub will have its own requirement on where the trap needs to be. While they're more expensive, most manufacturers make tubs in 6" increments, so there are 66" (rough opening) tubs out there. Personally, I find a stock 60" tub very uncomfortable and installed a 6' (72") tub when I remodeled...I would have loved one even longer. Considering how long a bath remodel usually stays around, that extra expense is pretty small for the increased comfort.
The drain diameter for a shower was probably 2", so is fine for a tub, but you'll likely need to tear up some concrete to get it where it needs to be. You need the drain where the tub you choose wants it, or it will likely smell and be very difficult or impossible to snake, if that ever becomes necessary.
A pro lines things up, and sets the tub. If done properly (takes some skill!), it can be done blind - i.e., with no future access, but you only get one chance to do it right - best left to a pro. If you do decide to go with a shelf or some such thing to fill out the distance with a 5' tub, pay particular attention to the waterproofing. Done right, it works fine, done wrong, mold/rot city! There are lots of waterproofing systems out there. Tile is NOT the waterproof surface; it's what's underneath that should be waterproof before the tile is even installed.