You can move the shower, the drain doesn't have to be centered, it can offset to one side but the framing would be a little weird unless you widened the whole bathroom. Either way that would allow for the extra space needed. Also busting up concrete and moving drains is actually very easy
just saying.
But if it was me I'd probably just make the tank 36" or even 30" deep to allow for the overflow, some plumbing and whatever else. Access from front is really for the best anyway. Every once in a while you'll need to scrap and clean near the seams of the glass, would be a nightmare from the back on that size of tank. Some stuff in this hobby is hard enough, doing it backwards doesn't seem like the smartest way to enjoy it.