Trans and rear end help

[biggpoppa3]

I just got a 1970 c10 with a 250 I6 and a three on the tree, I need to know what I could do to get it to run faster than 55 mph. I have a th350 transmission sitting at home that i plan on putting in it so I need to know if it will even bolt to the engine at all and how much stuff would i have to change to make it work i do have an aftermarket floor shifter for it. I dont want to swap the I6 for a v8 either I want the I6. Also im not sure what gears i have in the rear end but all I know is it is low so what gears should I use, i want to be able to run 70 - 80 mph easy so what gears and what could i pull them out of or what axles will interchange with mine any info would be extemely helpfull.