No adapter used on my 216 engine (original size in my 1951 3600). I did shorten the input shaft and throwout bearing collar to match the ones on the SM420. Lugnutz has some excellent how-to vids on the various options.
I don't have a tach, but according to one of the online calculators, with my 31" tall tires and 4.57 gears I was turning 2738 rpm with the SM420 at 55 mph. With the T5 I'm running (1352-222) and it's .72 5th gear, 55 mph is now 1948 rpm.
790 rpm less.
Calculator I used
http://www.blocklayer.com/rpm-geareng.aspx
