1961 C30 4sp Transmission: swap choices?

[MichaelNicholas]

Hello again.

I drove my new truck today for the first time (1961 C30 1 ton, 261 straight 6, 4 sp manual trans) and found a change I want to make right away: the transmission. As mentioned it's a four speed, 1st gear a granny gear, not worth using in normal driving, 2-4 are very short gears with a top speed of 55mph.

As of now, I'm planning to keep the inline 6, but am wondering if there is a common tranny swap that gets your truck a better range and a top speed closer to 80-85mph.

I'm having trouble identifying the trans based on the charts I have found on this forum and a google search. The Saginaw sounds like it might be common, but my inspection plate on the drivers side has 6 bolts instead of 7 (photo attached).

Thanks
Michael