help help??? 1987 TBI change over

[plowguy]

ok need some help did a tbi change over that I was told would be easy and now have a big problem ...
it started as a 1986 chevy k 20 with 350 carb

ALL I change over was from a donor truck was the TBI and computer, intake dist.. new howell wiring kit
however I did take the fuel tank from the 1989 donor suburban because it had the fuel pump and all (maybe this is the problem I was thinking same fuel pump but now maybe not!!

ok now it runs but wont go up a hill and it back fires out of tale pipe back when letting off of gas however it runs great in a parking lot all day like a brand new truck what did I do wrong??? starts right up runs great will do 60 mile an hour down the road if you go slow getting up to speed but when slowing down she backfires bad PLEASE HELP