Just an update, filled the truck since there's snow coming tomorrow and the extra weight will help. Looks like 12.6 MPG over 440 miles biased more towards around town than highway. Took 35 gallons so I still had over 20 gallons left or over 250 miles of range. IMO this is not bad compared to the 460 truck which would be getting 9-10 MPG in similar driving. I have a feeling the retarded timing was the major factor in low MPG.
I still have to clean the engine and check for any fuel leaks, but I can tell there are no major leaks. Feels like it has a lot more power when cold advance is on so I may advance timing from 9* to either 9.5* or 10* after I've had more time to establish current MPG. I've also thought about wrapping the up pipe and getting a turbo blanket. Not sure how that would affect MPG but I've read some that indicated is does make a noticeable difference in how quick the turbo spools. I'll probably wait for that until I find a ZF5 in case I need to change the exhaust plumbing to accommodate that. No luck finding a ZF5 so far, but that can wait until warmer weather since the truck won't fit in the garage enough to close the door and warm things up. When I do that I'll probably have the engine out for a proper cleaning, reseal, and head studs anyhow.