I was able to find mine on the build sheet behind the seat. Pull your cushion forward and look for a piece of paper between the springs. If it's there write down as much info as possible because it you even try to move it, it will fall to pieces. Mine came out in about 30 pieces! Also I believe the trim tag on the driver side kick panel has a spot for an axle code, some got stamped, some didn't. Here are the axle codes for 60 & 61:
1960 Rear Axle codes
Look up another year
Series Code Ratio Plant
1100 AA 3.55:1 Detroit Gear & Axle
1100 AB 3.36:1 Detroit Gear & Axle
1100 AK 3.55:1 Detroit Gear & Axle
1100 AM 3.36:1 Detroit Gear & Axle
1100 AX 3.08:1 Detroit Gear & Axle
1100 AW 3.08:1 Detroit Gear & Axle
1100 BA 3.55:1 Buffalo
1100 BB 3.36:1 Buffalo
1100 BE 3.70:1 Buffalo
1100 BK 3.55:1 Buffalo
1100 BM 3.36:1 Buffalo
1100 BH 3.70:1 Buffalo
1100 BX 3.08:1 Buffalo
1100 BW 3.08:1 Buffalo
1100 FE 3.70:1 Detroit Gear & Axle
1100 FH 3.70:1 Detroit Gear & Axle
1200 AA 3.55:1 Detroit Gear & Axle
1200 AB 3.36:1 Detroit Gear & Axle
1200 AM 3.36:1 Detroit Gear & Axle
1200 AK 3.55:1 Detroit Gear & Axle
1200 AX 3.08:1 Detroit Gear & Axle
1200 AW 3.08:1 Detroit Gear & Axle
1200 BA 3.55:1 Buffalo
1200 BB 3.36:1 Buffalo
1200 BE 3.70:1 Buffalo
1200 BK 3.55:1 Buffalo
1200 BM 3.36:1 Buffalo
1200 BH 3.70:1 Buffalo
1200 BW 3.08:1 Buffalo
1200 BX 3.08:1 Buffalo
1200 FE 3.70:1 Detroit Gear & Axle
1200 FH 3.70:1 Detroit Gear & Axle
1/2-Ton MA 3.90:1 Detroit Gear & Axle
1/2-Ton MB 3.38:1 Detroit Gear & Axle
1/2-Ton MC 3.90:1 Detroit Gear & Axle
1/2-Ton MD 3.90:1 Detroit Gear & Axle
1/2-Ton ME 3.90:1 Detroit Gear & Axle
1/2-Ton MF 3.90:1 Detroit Gear & Axle
1/2-Ton MG 3.90:1 Detroit Gear & Axle
3/4-Ton MH 4.57:1 Detroit Gear & Axle
3/4-Ton MJ 4.57:1 Detroit Gear & Axle
3/4-Ton MK 4.57:1 Detroit Gear & Axle
1-Ton PA 5.14:1 Detroit Gear & Axle
1-Ton PB 5.14:1 Detroit Gear & Axle
1961 Rear Axle codes
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Series Code Ratio Plant
Corvair 95 BL 3.89:1 Detroit Gear & Axle
Corvair 95 BY 3.89:1 Detroit Gear & Axle
1/2-Ton MA 3.90:1 Detroit Gear & Axle
1/2-Ton MB 3.38:1 Detroit Gear & Axle
1/2-Ton MC 3.90:1 Detroit Gear & Axle
1/2-Ton MD 3.90:1 Detroit Gear & Axle
1/2-Ton ME 3.38:1 Detroit Gear & Axle
1/2-Ton MF 3.90:1 Detroit Gear & Axle
1/2-Ton MG 3.90:1 Detroit Gear & Axle
3/4-Ton MH 4.57:1 Detroit Gear & Axle
3/4-Ton MJ 4.57:1 Detroit Gear & Axle
3/4-Ton MK 4.57:1 Detroit Gear & Axle
1-Ton PA 5.14:1 Detroit Gear & Axle
1-Ton PB 5.14:1 Detroit Gear & Axle
My bet is that you have a 3.90:1 rear end. It looks like the 3.38 option was pretty rare. Hope this helps!