I have an 82 K20,6.2 also...
You might have a restricted fuel line or filter,or a clogged air filter if its that low on power..injector pump timing will reduce power also if its off by very much,but unless you disturbed the pump,it should not have changed(there is a timing mark in the pump and the block where it bolts too--they should be lined up with each other)...my 6.2 is no ball of fire,but it will push more snow with less "pedal" than my 400 SB used too in my other truck--and it will never stall when I climb up steep snowbanks like the gas motor would!..I think it will run upside down if it had too..!
You have an automatic tranny??or a manual??--your truck might have "tall" highway gears,like 3.08's,that will reduce power a lot--diesels often used the high gear ratio's and the overdrive 700R4 trannys to reduce RPM's(diesels like to lug rather than wind out like a gas motor),also to improve fuel economy...
But put big tires on such a rig,and it will be a snail!...you probably would have to use low range to get up some hills off road if thats the case.
.You'll have to do some snooping and see if the timing is right,and you are getting ample fuel to the injectors,and the air filter is very important on a diesel--it clogs a lot faster than a gas motor one will,because the diesel sucks in as much air as a gas motor will wide open ALL the time,even at idle!...and they need lots of fresh air for good combustion...putting some fuel system cleaner in the tank wont hurt any either--the injectors on your 'used" engine might be dirty,that will reduce power and make it smokey..
