The only way to test in place is to pressure test it with about 3-5 psi of air pressure. If it hold pressure over night you may be good for now but still on borrowed time. Better off to just replace it. It could be pitted badly and still hold air but if you have one little pin hole the air will work its way out overnight. You don't want to have to come back a few years latter from now so best to change it out while in there. Even if it hold pressure it may be full of varnish. I have cut open good looking clean tanks to find a brown/yellow coating on the inside. You can run dual filters / water separators but better off to install a new clean un-corroded tank that you know is varnish free.

75-100 is what I was thinking.I'm not sure what size the current one is,I guess i'll find out when I pull it.The boat had a second tank in fro t of the console which I removed years back.I don't want to negatively affect the benefit of a big tank of gas(weight) on the ride by going too small on the tank.

In my case the tank is so long it comes all the way to the bilge, so it was easy to figure out the capacity L x H x W / 231. Because the tank is never always full, weight is changing as you use gas I would not worry about the ride too much. In my case I would like the weight as far front when sitting at rest. Going from a 144 to a 100 gal takes two feet off the rear of the tank I now have, which is around 250#.

Dug up a few more pics for you. Original tank to me had clearance issues with my 90* fill below hatch. Solution? Remove core of hatch. Just the beginning of your issues, pulling one hatch. Liquid Encouragement.

Third pic, Vezo Part I

Vezo

I don't understand that. When I got my tank from them, probably 20 years ago, I told them what I had and they already had the specs. Got it shipped to me (well to a local business) and it fit great. It was already primer painted and I also painted it with resin.

RDS has the exact drawings and plans for ALL of the fuel tanks they made for Seacrafts from 1968 onward. They are NOT, however, listed by Seacraft model. Tell them the date of your boat manufacture (not the model year!), the tank model number if still available, or tank dimensions, capacity, and fill-neck location and direction. They were able to exactly duplicate the tank for my '71 Seafari for me back in 2006, and also just got one this year for my '88 18'.

The 49 gallon tank (47 gallons usable) for my '88 18' retails for about $830.
It was a bit less with my West Marine Associate discount.

Yes Sir, I have had 9 tanks built to spec, over 3 years, for Jersey Speed skiffs from them. I had no idea that they had the original specs. I thought that Sunshine Tanks were SeaCrafts number one builder. I stand corrected. I just paid $1284 for my rectanglular 70 gallon, .125 Aluminum, 2 baffles, one fill, vent, pickup, sending unit, and coal tar epoxied. Yep. Florida tax, for South Carolina pick up and deliver myself...

Shop hard.

Michael.