There are a number of sources for a 29 NF. I bought one from Car Quest for my 56 Bird
NAPA # 7280
Duracell SLI29NF
Interstate 29NF-VHD
Car Quest (Advance Auto) 29NF40
The 29NF batteries have a somewhat low CCA, around 390-420.. This usually is ok but if you want one with a better CCA the Group 56 batteries have much higher CCA's
The 56 is almost the same size, should fit in the tray ok. It is a little shorter 2 ", a little less in height 1/4 "and a little bigger in width 3/4 ". I have a 56 in my 54 Ford, fits the tray good. I got these specs from measuring both.
Another manufacturer's sizes should be close but always measure before buying