When you said 4 speed I presumed that you were sourcing a truck 4 speed (SM420) with a granny gear which would require the DS and floorboard changes I referenced. The Muncie 4 speed they are suggesting is likely from a 60's passenger car. They are perfectly fine but would need a floor shift assembly and possibly a fine spline front yoke as well. You may also have an issue with seat clearance with the shift lever. Those Saganaw 3 speed transmissions are relatively cheap here - less than $200 for a good working unit. But of course you are not here.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine