Mike, wish I could help you with this but my Seafarer is a 29'. I removed my metal fuel tanks when I purchased the boat 12 years ago and replaced them with polyethylene tanks. Since my tanks are housed in a lazarette hatch separated from the cabin with a bulkhead and with independent ventilation I felt these low-permeation tanks would suffice.
Not sure where the tanks on the S38 are housed, but you most likely will have to go with permanent below-deck tanks as found on pages 604 & 605 of the West Marine catalog.
West Marine eCatalogs
A fellow Seafarer owner, Elan and Ashley are full time cruising on their restored S38C, s/v Silver Lining. They are currently in La Paz, Mexico and will soon be heading down to Panama.
I suggest that you try to get a message to Elan through his blog Sailing Silver Lining
Another couple options for advise is the Seafarer Research Center at Seafarer Research Center
Last on Facebook is the Seafarer Yacht Group. You will find a link to this group on the Seafarer Research Center's home page.
By far the Facebook group is the most active, but the Sailnet forum and the Seafarer Research Group are the primary web sources for networking about the Seafarers. There is also modest interest/activity at Sailboat Owners.com at this address.
Seafarer - SailboatOwners.com