I am not up on my 6BT fuel pumps but I do know a little about fuel pumps found in 4BTs.  I am not sure what pumps are in 6ATs though I would not be surprized if they were also Bosch models.  I recommend reading this little segment about lubrication in Bosch fuel pumps compliments of Steve Fox and the DTLC list. 

4BTs from early box truck applications have the Bosch VE Rotary pump while later models have the Bosch P1700 Inline Pump.  You might want to check out this page which compares Cummins engines and their outputs.  The VE pumps are easily tweakable by hand and I'm told any shade tree mechanic can do it.  But the VE pump can only be treaked to an extent while I'm told the Bosch P1700 inline pump can be treaked to allow a stock 4BT with stock torque output at 350 ft/lbs to put out in excess of 700 and 800 ft/lbs of torque, obviously at a loss of fuel economy. 

4BTs I personally have been looking into to purchasing are early models out of a Frito Lay van with the VE pump.  I plan on tweaking the pump for up to 130 hp and up to 350 ft/lbs of torque and no more unless I someday go to 1 ton Dana Axles.  

Lou Chou (see his site on Cummins repowers) and Alan Lungi chose to purchase brand new Cummins 4BTs which are terribly expensive.  Both run 1-ton axles with the P1700 pumps.  When purchasing a newer 4BT and typically brand new or close to it versions, you will get the P1700 pump.  I have called junk yards all over the country and have never found one that has the P1700 inline pump, only older 4BTs with rotary VE pump. 

The P1700 pump is highly tweakable, both Lou and Alan are pushing over 200hp with about 700 ft/lbs of torque.  Alan mentioned that over the entire course of the Rubicon he only burned 3 gallons of gas which of course totally impresses me.  The P1700 pump accepts most of the mods that the 6BT does, throttle plates, etc etc.  The difference is typically you have to have a tech familiar with Cummins engines and fuel pumps tune it or rebuild it.  However, Steve Fox says it can pretty much be done by yourself if you know what you're doing.