ntc350 bc cummins no power in high gears?

ok well shes getting olds heads,injectors and jakes all put back on with a inframe kit for the cpl 840 so I should be good, he also told me to buy all 6 piston cooling nozzles as well.

 

what injectors are suppose to go with that CPL...do they have part numbers on them. mechanic said its a newer formula model engine

 

my problem is tag is on engine but #'s arent visible anymore. also im unsure if engine is the original

 

well engines been overhauled truck still runs the same way. i had a idea could the trans and rear combos be playing a role in truck having no power. it has a rt12709R trans (which for the record i dont know what the "R" stands for and eaton 3.90's on a 11r 24.5 i had a mechanic tell me my problem is the trans is direct making engine work harder in last 2 gears 8th and 9th to accelerate since there so long. what i did notice is this when i throw 5th gear and run it up to 2000-2100 rpms it then shifts into 6th and bogs motor down to 1300 rpms into high range WTF? then when you throw 7th to 8th gear i have to run all the hi range gears 6,7,8 &9 to 2000rpms so it will keep engine in power band but still drops to 1350-1400 rendering 9th just about useless until i catch some down hill speed then i can throw 9th otherwise i would have to hold 8th hi reved at 1800 till i catch a hill to throw last gear. this is only when truck is loaded 45k load only weight. its just ridiculous to think that after new pump,lines,filter base,overhaul,new heads,remaned injectors,good turbo,new turbo connectors,good aftercooler,fresh heads, and overhead gone thru truck still runs like a UD cabover pulling 80k. ive put alot of money into this old fart and i dont want to sell it but if i cant figure out problem soon imma have to let it go. if its the trans then i will try to get mine converted to OD which my friend said would let truck run out better thru gears i thought it would be the opposite away around hes says otherwise. why else would truck run awesome light to light but hi range take forever?
when im pulling something but Empty it will catch speed about 5mph more then top speed is not loaded. on a side note this is original combo on truck 350bc4 ,rt12709R,3.90 but it seems it was originally on low profile 22.5's and now its on 11r24.5 could this be my problem when i got it, it was on low pro 24.5 but i never took it on eway i went directly to install new taller tires. also i dont think it matters but the truck i think used to be a single axle and it had a newer tandem installed with metric hubs that are also 3.90's would it being made a tandem give it this problem of not pulling with this combo? im stumped sorry for the long novel.


also truck never leaves miami whats the overall take on removing the thermostats on my engine? im constantly on my guages and fan clutch switch if needed.

i really do thank you guys and all the input.

 

i guess no one knows, off to the selling block it is