To me it boils down to how much time you got?
i think you should, depending on how much time you got, experience/knowledge, and budget, go with whatever engine you like the most.
But to answer the question, and if i were going to do some occasional towing, which i'm not, but to answer the question 350 crate is the easiest, cheapest, fastest way. (and i wouldn't mess with aftermarket EFI, because why.....? Quadrajet or equivalent new carb and mechanical fuel pump is like $400 tops, Aftermarket EFI minimum is $1000+.......)
This is literally a bolt in affair, and i've done it enough times where i can have the truck running again in probably under 8 hours.
Now, i will acknowledge that with an LS engine, you will most likely get 2-3mpg better than a smallblock(not to mention numerous other advantages). But there's no way i could get that running in under 8 hours. Probably would take weeks if not more due to the computer and screwing around with the fuel pump which has to be high pressure electric.