I am a 4 speed guy... lots of expriecnce, and I build them...
The best real world affordable 4 speed I have found is the Borg Warner Super T10 used in 1975-1981 unmentionables.
The trouble with Muncies is the gear ratios. A flathead has a wonderful practically never ending torque curve, so you don't need that goofy close ratio crap to stay in an inconvenient power band.
Muncies are available in a 2.54 and 2.20 first gears.
Niether of which is really suitable.
The BW is most commonly found in the unmentionables in a 2.43 and 2.64 1st gear. These are identified by 2 or 3 "rings" around the fine ( 26 spline ) input shaft.
For a really "built" flathead, that "3 ring" ( 2.64 ) transmission would be killer in a car up to about 2600 lbs. For the heavier full body cars like my favourite, ( the PHAT ones ! ), there is a 2.88 first gear ( 4 ring ) which can be had. These transmissions are available in 5 & 6 ring ( up to 3.22 1st ) ratios.
Light weight aluminum case, the low 1st gears isn't just a lower first gear, or like the Muncie where they just spread out 3rd & 4th. The gear ratios are spread out
They're stronger than a Muncie, and virtually indestructable behind ANY flathead, Richmond makes every single part for them, including improved brand new units.
Standard Hurst shifters bolt right on.