The RPM won't care if you have a 350, a 235 or an electric motor. The rpms are all a matter of your transmission gearing (final ratio will either be 1:1 or something lower if you have overdrive or a transmission geared for final overdrive gearing), the diameter of your tires and the ratio of your rear end. And when you start calculating overall gear ratios, it is instructive and important to look at what GM engineers thought you needed for all your gears. In general for a 1/2 ton pickup with an inline 6, your first gear total ratio (calculated by multiplying your first gear ratio times your rear end ratio) ought to be around 12. With a 350 you can make do with an overall ratio of 10 or possibly even 9 if you don't plan on hauling a lot of heavy stuff around. Think of it this way: if you have an inline 6 and your first gear ratio is around 2.94 and your rear end ratio is 4.11, then your overall ratio will be 12.08. A ratio of 12 will let you load 1000 pounds of stuff into your bed, add a passenger or two and drive without burning up your clutch and you can even take off on a hill. If you have an overall ratio of 8 you can put an empty ice chest in your bed and that's about all you should try to haul around. A more powerful engine lets you lower the overall ratio, though. More torque, more horsepower will do that for you. Then in second gear, your standard AD truck transmission had a gear of 1.68. This times 4.11 gives an overall ratio of 6.9 but remember please you're already moving when you start using this ratio. And your final ratio is 4.11 (the ratio of your rear end since your third gear transmission ratio is 1.1). Now if you had an overdrive transmission back in the 1960s, your overdrive ratio would have been .7, so you could multiply that times your rear end to see what your final rear end ratio would be...2.87 or so. With transmissions there will be different ratios for each gear. Friends of mine have run out and bought a 5 speed transmission without ever thinking about the overall ratios they would have, nor their rear end ratio and consequently have made their vehicle run much worse as a result. Consider in the T5, your first gear could have been 3.50, 3.76 or 4.03. If you had a rear end from a Camaro, it could have been a 2.77 ratio. 2.77 times 3.50 = 9.69. Almost worthless for a pickup. Ok for a light car with a very large engine.
As for your 350, those came in varying degrees of power and torque...ranging from not very much to a whole lot. What sort of gearing will be best for you depends on what you have there (which I don't know), but I'd say an overall first gear ratio of 10 would be ok if you're not hauling a boat, trailer or bed full of firewood. Your 350 might have only about 140 horsepower (which is about the same as the 235, give/take). If you can fill in more of the variables I or several other folks here can give our thoughts and suggestions. Also depends on whether you want to end up with a street rod that 2 or 3 people can ride in or a working pickup.
