Re: 1973 starcraft ss restore in progress
Personally, it'd come down to time vs money & how you boat.
No time to hunt & search for a below deck tank that might fit, rework the below deck structure to allow it to fit, or to rework the above deck structure to run the fill hose, but you can afford a custom, go custom & put the tank where you think it'll serve you best.
Have time & don't mind the hunt, and possibly save money, start searching for a possible tank online/CL. Donor boats often provide motors & tanks. Might get lucky, or at the very least find a donor to part out & fund the custom tank you'd really like to get.
If you're going to go max hp w/ the motor (most 120hp motors will outweigh most 70hp, both 2cyl or both 4cyl), you regularly fish w several hefty friends, generally boat w/ about max capacity for people aboard the boat, and/or take lots of gear most of the time, moving weight forward (batteries too) will hopefully improve the overall handling & performance of the boat. But no guarantees.
If it's usually you & the Admiral out to wet a line, or just to cruise, and a 70-90hp motor will get you where you want to go about as fast as you'd like to get there, putting the tank back where it was likely originally, could be a much easier solution.
There are lots of decent sized above & below deck tanks available, by Moeller & others I'm sure. You might just try to reorganize the below splashwell space, get a tank that runs across most of the beam of the boat, has a capacity to fit your general needs and then move the batteries to port (counterbalance your weight when solo).
Not much 'need' to have 19gal of fuel aboard in WV where I'll be boating, even on a gas pig 50's FatFifty, so generally a 12gal would work for me.
I bought the bow tank because of it's shape, it was cost effective, and I have several rigs to choose from to put it in. Likely won't be the 15' Jet though. Tad heavy when full, IMHO, for the bow of a 15ft boat. But you don't always have to run the tank full, some probably don't, esp when you're filling more then 12gal at the dock/marina.
Hope that ^^^ helps.
For deck/under the splashwell space & how I think I'll use the boat, I'm very interested in below deck tanks.