Need help boosting my MPG's

You bet every time CAT or Cummins. I have had to straighten many a 400 BC out after being in framed the fuel smoke at start up began.

If a guy would look at the detailed bill he could at least see what piston and injector were used, make sure they match and know what to set time on from there. But that still doesn't tell you what was taken out of it.

 

I was reading something somewhere a while back (might have been on heavy equipment forum) about a guy with an old 3406 (A?) with something similar. Can't remember exactly what it was but I think pistons from the 310 horse arrangment were combined with pump settings and timing from the 375 or 380 horse arrangements.

As for S/N check on intake side of block, around #5 cylinder. My old A block has it stamped there and no where else.

 

Pretty normal, does it idle up after a cold start up till oil pressure gauge starts to move. Maybe just a 100 rpm or so? It should usually

 

I do not have any Cat info but some folks do. They would need to know the replacement piston number which could probably be retrieved form a kit number if you do not have a piston number. Then the injector number to see if it is correct for the piston, there could be many.
The block is just the carrier for the guts. Even if you had the block number that does not guarantee they belong together. Custom tunes in the past had nothing to do with an ECM. That is why I put happy engines back just like they were.
There is a lot to other peoples good posts here and you had it bare blocked. Some times Cats have timing advance issues. Some engines have offset crank and or cam keys. If they were replaced with a straight key or put in backwards your timing is out of wack and your not set up correctly as some have suggested.
Some posts have been made about newer engines being reprogrammed for low NOX and and messing everything up. Hope that has not backed up to mechanical engines. Required by law, dealer does it, and acts like they know nothing about it. Was the cam replaced?
That's all I have but if you fill in the blanks maybe a Cat wizard can had you out. If I were you I would feel like it is the responseabilty of the shop that did the work. Good Luck!