5.9 slt plus Durango donor for 74 duster

magnumswaps.com has pretty much every piece of info you need and can be used to calculate a parts and price list

Since you already have a manual trans, I'd consider an 833OD. They can be had a lot cheaper than a "regular" 833 and have roughly the same gear splits as the 3 speed.

The OD will surely help on MPG with that 5.9.

...and no trans tunnel mods!