Build the 261 and buy the adapter that lets you run the 216 valve cover. Replace the rear gears with the readily available 4.57 gears from a 3/4-ton or the 4.11 gears if you can find a set.
That would give you an original looking truck that should easily do 55 unless you do something crazy with the tires.
What he said. Remove the 2 rocker studs that hold the valve cover on the 216 and replace them with the bolts in that location in the 261 to use the older valve cover. If you want to build a hot 261 I suggest checking out and even joining Inliner’s International. Here’s an article from there that will give you a good start:
http://www.inliners.org/tech/261_build.htm
