The 66 are made for 38 specials. The 73 rifles can also use 38 Specials with no problems. Some of the early imports (over 10 years ago) had elevators that we're quite beveled enough so some preferred longer cartridges, but that can be fixed easily.
I know a lot of folks, including me, are shooting 38 Specials in their 73 rifles.
A lot of stock Marlins and 92's do a little better with longer cartridges. I loaded my own 38 Specials, for one of my 92 and my Marlin 94, but crimped in the lube groove and they worked great. My other 92 shoot 38 Specials just great anyway, but it varies with the rifle.