For those who want a reliable battery solution but not so much concerned about originality, here's one for the front battery mount trays in '65-'66 GT350's: Use an AGM H5/Group 47 that has the side mounting "ears" like our old Group 24F's used to have. The H5 will fit the tray but is shorter in length, just short enough to change the position of the mounting bolt in the tray to the Group 22F position. Obtain a Group 22 mounting bracket and you're in business. The + and - posts are in the correct locations and most H5 batteries will have at least 600 CCA or more.
Thanks for the tip Stan!