Brownies typically have a low gear, direct and OD gear, some have 2 lower gears which is OK, not have to use lowest gear, when you find one, just enter the MN to get the ratios, I. E. 5831 box, ratios available. Most only care about the OD ratio.

When with a load, best go to high direct, then OD, back down to high direct. Then lower, keep big loads off OD ratio to save ear on cluster gear. These are not weak trans'.

There is not a 'simple two speed' 'brownies' only to use direct and OD without a load.

Mico Loc type park brake units are not legal, although many use them. Best to use a real park brake, even if only on main trans.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.