T-Bird Battery

[Milt K from Pa]

Barners,
My battery in my '57 Bird died yesterday even with a Battery Tender on it. I used it a week ago with no problem. Problem now is finding a new one. The size group is 29NF and the only brand I can find is Interstate, same as in the car. I'd like to put an Optima in, but the sizes are too wide for the tray. Anybody have a strong replacement??
Size is 12-1/2 X 5-1/2 wide X 8 high.
Thanks for any help.

Milt K from Pa