On the trans thing thats good if your ok with current one, wasnt sure if you wanted auto for a particular reason aside from gearing. Rear axle ratio is what mainly holds these trucks back from factory for modern interstate speeds, the deep 1st in trans can overcome taller gearing though if that is something you need.
If you can find someone that rebuilds the Qjets I really like them for the balance between mpg and power. If you cant find someone local or don't want to try it yourself then summit racing carries a rebuilt one for $240 basically.
https://www.summitracing.com/parts/urm-3-3256