I'm torn... "sport wagon" elicits comparison to the regal "sport coupe" which was what they called the ttypes of the era. Even the lesabre sport coupe got the carb turbo engine. So it seems like a turbo 3.8 is what belongs... or maybe a stage 2 4.1 with the repop gnx rear differential cover/bars in back since we're spending other people's money....The way I look at it. It will never be original again. that's why I'm a proponent of keeping it original. However, I could see a drivetrain upgrade, and yet keep the exterior original in appearance. Like James, I'm a resto-mod fan. It would be nice with a GN drivetrain.
But 70s... wagon... screams big block to me.
Keep the paint and interior original. Don't cut the floor and switch shifters. Just drop a big old torquey buick motor in with the fattest tire that fits without a notch or rolled lip and flog it.