Rear shocks - '63-'66 vs '67-'72

[Greg63]

Last week I switched out my rear coils and shocks in my '63. All went well. I took the truck for a drive and it seemed to be very bouncy at first but settled down after a bit of driving. I was supplied with Monroe Matic-plus #32301 shocks and Moog 6101 coils.

I just referenced the part numbers on the internet and found that the coils are correct for '63-'66 1/2 tons with coils but the shocks are for '67-'72 1/2 tons with coils. Monroe lists #31000 for '63-'66 rear shocks.

I was always under the impression that the '60-'72 rear suspensions were all alike (generally). Why would there be two different part numbers for the different rear shocks? Should I return the '67-'72 rear shocks?
Greg