I read the statement below on another Forum and thought i would share it I have not found where Diesel power qouted this but if it is true I thing a big block gas to a duramax would be the way to go.
Next thing to check would be how to keep the motor at low rpm for fuel consumption.
as quoted from diesel power
"For decades, every Chevy V-8 (with the possible exception of the Indycar engine program) has shared the same six-bolt transmission-to-engine bellhousing-pattern designed for the original 265ci small-block Chevy engine back in 1955.
The upside to this commonality has meant that all Chevy V-8 engines; small blocks, big blocks, 90-degree 4.3L V-6 engines, 6.2L and 6.5L diesels, LT-1, LS1, 8.1L, and even the 6.6L Duramax will all bolt up to the same transmissions. This has helped make Chevy engines the most popular engine to swap into other vehicles."