the problem with a 99 is what injector to use. people use 784s because they clip in but the injectors really are too big. WOT will be rich and so will start up. closed loop will be fine since the ecu will pull the fuel but open loop will be rich. 784s flow right about 25lb/hr at 49psi

there are two other choices

1. the ford 19s, these flow about 21.5lb/hr at 49psi which is under your 22.5lb stock

2. the chevy 25lb LS1 injector. these are rated at 58psi and flow 22.9lb/hr at 49 psi making them a very close match

what i don't know is how easily the ls1 injector will fit in the truck

but i can do math on injector flow rates for you

be careful if you buy ls1 injectors. they change through the years and some of the larger flow another 3lb/hr of gas. you would want the early 25s not the later 28lb units

 

I thought 784's flowed at around 22.65 lb/hr at 49.2 psi, with 21.3 lb/hr at 43.5 psi. (?)

 

buy 6 measuring cups
remove fuel rail with injectors
put cup under each injector
"start" engine a few times to check fuel delivery.

 

depending on who's testing it's 21.5-21.9@39psi which is 24.5lb @ 49psi

this is why it and it's obd1 twin the 703 are such a good fit in the 91-95 cherokees which use 21lb@39psi injectors.

still, guys use 784s in your situation, you may of had a clogged one? who knows. if these were junkyard injectors i would send them to http://fuelinjectorconnection.com/ for cleaning and a flow read at your pressure to see what you're getting

you can do a basic test with the method stated above