New tranny means new driveshaft and rearend too. These trucks have closed drvelines. Must be converted to open driveline to start changing much around. Open came late '55, closed before that. Thankfully mine is a '54 so the rear end has a 3.90 in it. Earlier ones had 4.11 or worse (for higher speed cruising that is). Mine does good at 50 mph or so. If you plan on having a need to go faster than that, you will have to consider all this and better brakes as well.
