4.2 is a inline 6. and i don't think it was a bad motor however it isn't a popular one; it was phased out, probably because it had same power and gas mileage of the LS series V8s and didn't make sense to have the rare I6 in production because it only came in a few vehicles.
aka its easier to put minivan motors or V8s in SUVs; no need for a separate line of motors.
cool swap, but i don't see any benefit over an LS 5.3 unless you have a free driveline donor.
The 4200 (yes its an inline 6) engines were built upto 2009. Main reason to use them vs a 5.3 is:
1) weight - the 4200 is ~ 150 lighter than the LS.
2) its easier to bolt in another inline 6 than do an LS swap. Less exhaust to deal with.
3) starter is mounted up high
4) with a free flow exhaust power increases ~20 HP - to ~ 300HP.
The engines take well to a turbo - on a stock 4200 - add 8# of boost and your at close to 400HP and 380 FT/Lbs.