I'm trying to find out what battery group is needed for my 1961 Olds 98. When you search online, it comes up with a Group 27 or Group 24 as an alternative. Well I tried the 27 series battery from work and it's way too wide. The battery tray is just under 6 inches wide from what I measured. Also if someone has a picture of how the battery holds downs setup, that would be very helpful.

 

The original battery was a Delco R63, which was a Group 60 battery. I doubt those are even made anymore. The Group 60 dimensions are 13 1/16 x 6 5/16 x 8 7/8.

 

BCI Group 60K. Long obsolete and available now only as a reproduction.

There are some modern batteries, Group 51 comes to mind, that will work.

Get yourself a 1961 Olds factory shop manual. It will show battery mounting.

 

There's this one;
Oldsmobile Antique Auto Battery (1954-1965) S60KL - BCM (batterycentralmall.com)
Pricey, but it is a no hassle fit. Looks righteous too.

If you want to go on the cheap, a group 27 is probably closest to a group 60.
Expect that the hold down will not fit without modification.
And you may need a longer battery cable.