The 292 will have enough torque, altho they are not the best engine for going 70 MPH for long distances, unless you have an Overdrive. The Small Block should do just fine and even tho the Big Block will give you torque and power, I'm not a big fan of them in trucks.
Altho not original to your truck, a GMC V-6 will give you plenty of torque, altho again, parts are getting hard to find for them.
And lastly you could consider a Detroit Diesel. The 4-53 was available in some '60 to '66 trucks.
All in all tho, my personal pick in your truck would be a fresh 292.
Bill
