I have a 1999 s10 Blazer that I did a v8 swap last year in it when the 4.3 finally give up. I got a crate motor for a 1998 k1500 and had the computer reprogrammed for the 1998 k1500. The wire harness was sent out to be modified so I just had to plug it back in. After getting the engine running I have been having some running issues. With the stock mass air flow sensor for the 4.3 it would run good, but would throw codes for the bank 1 and 2 running too rich. When I switched the mass air flow to the stock 5.7 sensor it seemed to lose power and still had codes pop up but for banks 1 and 2 running lean. I got an aftermarket mass air flow sensor from Jet Performance to see if it would make a difference as well as getting a programmer from hypertech. It still is throwing the code for the rich/lean condition. Looking at the fuel trims at speed both banks are sitting at nearly -30% fuel trim with the aftermarket MAF. I placed oxygen sensor extension on it as I heard sometimes by switching to headers it can throw off the o2 sensor calibration. The sensor extensions helped a little mainly when I put back on the 4.3 MAF (had fuel trim move to -20% for both banks) but it still has a pending code. If anyone has had any kind of issues like this doing the swap and came across a solution I would be very thankful.

More info on the truck:
1998 k1500 vortec 5.7
k1500 truck programming (deleted evap, egr,and post cat o2 codes)
Shorty headers
Stock vortec intake
hypertec max energy programmer
Solid axle conversion (dana 44 front 9" rear, 3.5 gears)
35" tires
new distributor
new ignition module
new crank position sensor
jet performance mass air flow
new injector assembly