I think if the only thing changed is the engine (keeping the 4 spd behind the 617), your MPG would not be what it was with the 4 cyl. I am not familiar with the OM617 numbers, but if it is a turbo, you would need to change the rear end ratio (higher=lower numerically) to get anything close to 30 MPG. (EDIT: just re-read that the donor is an '85, so it is turbo)
If you are close to CA, I could use a 240 bottom end!

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All Diesel Fleet
1985 R107 300SLD TURBODIESEL
2005 E320 CDI (daily)

LOTS of parts for sale!


EGR block kit
http://www.peachparts.com/shopforum/diesel-discussion/355250-sale-egr-delete-block-off-plate-kit.html

1985 CA emissions 617 owners- You Need This!
Sanden style A/C Compressor Mounting Kit for your 616/ 617 For Sale + Install Inst.

Sanden Instalation Guide (post 11):
http://www.peachparts.com/shopforum/diesel-discussion/367883-sanden-retrofit-installation-guide.html