If tanks are all you have to replace, go get the boat. Dimensions be damned. I think all the 28 year models and perhaps some within the same year varied slightly. It also depends on how you want to equip it now.
Ours has the pump-out tank removed, we use a toilet that we can haul out and empty in the marina lutrine. We just have a freshwater tank, located under the seats on the starboard, near the kitchen sink, and the gas tank, located behind and above the engine (Atomic 4). If your boat has been converted to diesel, chances are the tank was changed.
The original tank was a steel 8.5 gallon. Ours is going to be replaced soon, it's a rusty ugly old thing. I believe there's a stock plastic 13 gallon that will fit in it's place avail from West Marine. One note...without a gas gauge, it's a bit of a pain to know how much fuel you have left. One way to solve this is to thread fittings at the top side and bottom side of the tank, seal them in well and run a piece of clear gas line between them as a fuel level indicator that should be viewable from the gunkhole access door that leads to the tank and engine compartment. Alternately, you could mount it on the other side and crank your head down under the cockpit seat to view it also.
Figure roughly 150 for each tank and you should be in the ballpark. I wouldnt think tanks should be over 500 lock stock and barrel unless you have needs far greater than ours.
BTW, ours is a 77 R28 tall rig.