66 C10 w/250 Transmission Upgrade

[ADickson]

As the title suggests, I have a 66 c10 with an 250 inline 6. I intend to keep the stock motor but I want to upgrade the 3 on the tree to something that will permit interstate cruising speeds. It should be known that I'm less than mechanically inclined but rarely too skeert to try something new. I'd prefer manual (5sp?) but not opposed to automatic. What options do I have to, more or less, bolt in an upgrade with minimal fab work?

So far all the recommendations are to return my crate motor and get a 350 with an automatic transmission. Hilarious, but not helpful. Hoping someone here will have a solution.

I appreciate your time.