[71cadc10]

I went thru this recently....I ordered Gates 351160 seal kit. This kit is listed for Saginaw pumps back to '68 and also newer LS pumps from 2000's.

It has all the seals in it, for a rebuild. I only used the o-ring for the large fitting out of the kit, leak stopped. the kit is under $7 from Rockauto.
The o-ring is critical for sealing, and if it's old and shrunk up it won't seal. It should be somewhat tight and you should feel it compressing when you screw it in the pump housing.