As the other guy pointed out, the sherman probably won't help with brush cutting, as it slows the engine to tranny connection, as it is a pre-tranny.
If you slow the tranny to lower the ground speed, you lower the pto rpm. If you want a lower ground speed and the same pto rpm, you would look for a howard tranny.. they were used to slow the N down to use a rototiller.
However again, as pointed out.. 1st in the 8n is fine for brush cutting.. a halfway decent 8n can pull a 5' brush cutter.
The early 8ns.. up to about mid 52 had a high reverse gear.. in mid 52 they went to the lower reverse gear.. you can tell if the left side bottom of the tranny has a bulge cast into it to accomodate the larger reverse gear.
As for the NAA.. most came with a 4spd similar to the 8n, however there were 5 spd upgrades available.. and yes i agree.. the N's are geared pretty high.
I don't know the exact speed, but over 1st is faster than second.. etc...
If you have a proofmeter on your 8n ( 50 and up? ) then it will list ground speed. Rope off 52.8 feet, and run at a set speed/rpm that distance, and have someone time you... verify that your proofmeter is accurate.. ( that 52.8' times 100 equals 5280 feet.. 1 mile.. figure your time to run the course in hours.. i.e. 1 minute is .016 an hour.. 45 seconds is .0075 hours, etc... multiply that by 100 and see what you get.. I'm gonna guess 1st gear ground speed on an N is somewhere between 4-5 mph at pto rpm...
now check the time and do the math on the under and over setting.. that will tell you your speeds...
Soundguy