ok so this is gonna be a lil long im stuck and i really wanna get this issue fixed

i bought my 1988 silverado 2500 350 2wd for 1200 from a friend of ours body was in amazing condition and supposedly she had a rebuilt motor put in and had only put 8k miles on it so i figured this was an amazing deal. well since i first drove it home i really have never felt the power of the 350 i thought the truck should have. well i started doing the typical tune ups anyone would do when you first get a used vehicle. changing filters, spark plugs, running carb/injector cleaner through the tank and after i ran the injector cleaner through the idle started fluctuating up and down and sometimes it dies cuz it goes down to far. i have changed the tps-throttle positioning sensor, egr valve, spark plugs, i did some research on the brain box and found out that the service number i needed was 1227747 but i had a brain box out of a 88 s-15 for a v6 motor but when i put the proper serial number brain box in the truck it wont stay started at all it will start and die instantly. please throw me some ideas that i could look into to get this fixed. im so lost as to where to turn next. pls any ideas will help.