Thanks for the plug, Bjorn.

Here's a thread on the Pontiac OHC-6 site that discusses the MPG you might expect from an OHC:

OHC Mileage

The change-over to an OHC from a V-8 is a little involved. The TH400 will physically bolt to the block, but the list of stuff you need for the conversion is pretty extensive, including some parts that are almost impossible to find, like OHC motor mounts.