When I decided to put in a perminant tank I spent a lot of time thinking about the shape and size I could use. Too small and there would be no gain, and too large and there would be no room in the locker for anything else.  I chose the 16.5 gal tank in the configuration shown below. other parts needed were:  deck fill, deck vent, fuel guage, fuel hose,  tank vent hose,  3" vent fan,  vent grill, #8 screws, teflon tape  3" vent hose, 14 guage wire, tank tie down straps, 3" hole cutter, stainless pipe clamps.
Another consideration though I don't know how much good it will do, the 77 C25 has a little too much gear on the port side and without a little creative ballast she tends to list to port. The new tank will add to that headache but my soultion has been to go the the gun outfitters as buy 25 lb sacks of lead shot. they are easy to place and to move. 
Hmm now let me think .....
Tell me this is going to work.....
The tank shelf in the 77 makes the actual opening into the locker very small
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