Got started on the project this week. Pulled the slant6 and trans out.
Going with a Cummins 4cyl turbodiesel, and TH400 trans.
Trial fit of the Cummins shows it almost sitting right on the /6 motor mounts. The bolts line up and start in the holes, but the pan hits the crossmember and steering draglink. It looks like the mounts can be moved back a couple inches, I'll try that and see what happens. I may need to raise the mounts up a couple inches. The other option is getting the proper mounts from Dodge, but what fun is that?
Amazingly the rubber motor mounts appear to be the same on both motors.
I'll get a couple pics when I test fit the motor again.

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'81 D150 2WD shortbed 4spd OD.

my dad be proud of you...he always wanted to put a kittycat in a pickup but they dont make one small enough...he runs a deisel ford pickup...he repaired most the equipment used to build east coast interstate and then worked for the biggest cat house in LA county,when on to be tech advisor befor retiring ......good luck on project

keep on roddin'

sounds like fun

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Nothing beats a slant six
1987 D150