Keep looking at Moeller plastic tanks. Somebody here found one to fit, I think, maybe it was smaller, no v belly???? The 55 gal sure looks close if you can get one with the fill on the starboard side. I just noticed the make a 032555 and a 032555-1....maybe the dash denotes the side of the fill? 51"x 25.5" x 15"
Try
http://www.speedytanks.com/ aka Atlantic Coastal Welding in Bayville, NJ. They made a replacement belly tank for my 84 V20 cuddy (should be identical to yours) many years ago for $500. Excellent quality, better than original. Dunno what it would cost now
Shipping is going to be a major cost.