I dunno man.. I got to tell ya I don't have a bit of problem with the splash oil 6's and i've in the past had fantastic experiences with the later 261 inline 6.. a real workhorse.
The words i'd use to describe my 6's would be anything but 'weak'.. my splashoil 235 in my 46 2 ton has enough low end grunt to change the rotation of the earth.
But a lot of that has to do with gearing, and it's geared lower than that dude from the Oak Ridge Boys can sing "Ohhhmmmbobba bow bow".
The trade off with that kind of low end torque is you give up high end speed (my top speed is in the 50-ish range)
But in a 66 we're talking about a much more modern 6 and gearing.. but keep in mind, if your rear and tranny is geared low, you could put a 454 big block in front of it with a blower hanging out of the hood, and you still ain't going to go any faster than what you can go now.
So stick with the 6 or go V8.. the up side is engines like 350's are a dime a dozen, and parts are cheap and easy to find. You'll likely loose low end grunt if you swap out the rear and or tranny, but if you're not pulling stumps with your truck, then that doesn't matter.
The I6's are definitely easier on gas than a 350 V8, and so long as the engines aren't just beat, have ample power for daily driving, light towing, and general purpose truck use.
So if you want a 100mph tire smoking hot rod.. go V8 and a slushbox.. if you want a truck to use as a daily driver, and aren't worried about running low 9's at the track, go with a rebuilt I6.
