GM offered a Saginaw trans with a B/W OD from 66-69. They work great and you don't have to modify anything from what you now have. They are more difficult to find than the early OD's but still a lot less cost than a Gear vendors.
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