[reelapeelin]

Chris, I hear what you're trying to do..and w/the advanced plannining you're putting into it, it should be doable...the best thing you can do is get your dad to pull the inspection plates over the tank and see if he can find the mfgr's sticker on the tank...if so it will have capacity, model and serial numbers the will be more likely to insure the right tank is waiting for you in Bluffton...
Does the tank hatch-lid go under the steering console?...if so, then I don't see how you can do it w/out pulling the console...
Look in the Links Section of this forum for fuel tank mfgrs...I'm pretty sure the original manufacturer of your tank will be in there...the sticker I mentioned should, of course state that as well...on my cuddy, I can read the tank's sticker and hopefully your dad will be able to see it, too...NOTE;...if the tank goes under the console, he may have to get under there to see that sticker...
Unless all the filling, vent and supply hoses have been recently replaced, it'd be a good idea to replace all those while you're in there...10 micron filter should be added as well...there are also TWO ground wires you need be sure are intact before finishing up...one goes to the NEG battery and the other goes to the filler cap...use pull-cords when you pull the old filler line and vent lines out...you'll need 'em when putting the new hoses AND wire back...
SEARCH this forum for fuel/gas tank replacement threads...LOTS of discussions over the years about this issue and you should be well versed after reading through those...
I didn't have to pull my tank(WHEW!!), but when putting the hatch-lid back, had to provide new material under the floor's lip to catch the screws as the holes had wallowed out on some, so be ready for that...no big deal, but takes time if you don't anticipate it...
Good luck and let us know how it goes...I'd ask, but you ain't gonna have time for pictures...