While I understand your situation, Pigpen not having a diesel under the hood would be as wrong as two boys ****in. I mean, it just aint right. Find a 6.5 turbo and a 4l60/4l80 and keep wheelin.
In all reality, 6.0 or 454 w/ a sm465 behind it and 203/205 doubler. My choice would be 454 w/ da pane (just because I don't want to catch the FI gheyness fred ). The 454/465 will be easy to source parts for atleast.