That was an 89 Tuned Port Injected 5.7L IROC-Z,
I owned that more than 12 years ago. They were 50 state legal emissions compliant and had No gasket or seal issues, unlike the factory manifolds.
The JBA headers are marketed as direct drop in and hook up to the factory exhaust in place of the factory manifolds, in the Jeep Liberty. They have been successfully installed in some of the earlier XK's and Wk's.
These are the stubby headers, with the same outlet location as the factory exhaust manifold? Thus they are a direct bolt in?
Those really aren't "headers", but yes a quality built pair would bolt in without major modification and might NOT leak, depending on the quality.
It depends on the vehicle/platform and the quality build of the stubby header. Pacesetter made a stubby header for the Neon R/T's, in that case, the stock exhaust manifold flowed well enough that the stubby header was barely, if any, improvement, but pacesetter's low quality resulted in leaks, cracks and rusting out. They produced an upgraded exhaust manifold for the neons that elminated the cracks and it was just smarter to go with the upgraded stock manifold then the pacesetter header, that although it was easy to install, didn't produce any extra power and would just fail much earlier than the stock part.
I would check the part number for 3.7L exhaust manifolds for the Liberty and the Commander. If both vehicles have the same part number for exhaust manifolds, and the replacement part is designed to bolt in without modification, then if the original part works in both vehicles, logically the replacement part would work in both vehicles as well. Usually the aftermarket seller would take the time to figure that out and list as many applications as possible to make as many sales as possible, so I would NOT assume that just because Liberty and Commanders have the same engine, the same header will work for both. Of course the Commander may have engine bay walls and other equipment that might interfer with the Header, that the liberty does NOT and thus it won't work
A real "header" has tuned length runners designed to create a scavenge effect on the exhaust and will result in more power. That means the runners are much longer than stock and thus the connection point to the rest of the exhaust is very different.
The stubby headers, may reduce flow resistance and that would give you some power increase if the rest of the exhaust and the intake side of the engine supported the additional flow.
If you put the stubby "direct bolt-in" headers on your Camaro, I can see how they never suffered any leaks or problems. If the factory exhaust manifold was the only choke point on air flow through the motor, I could see how just a set of stubby header that flowed a little better did make a big difference in power. In another vehicle/engine, where there may have been several chokes points in the intake and exhuast system, swapping in a stubby header may NOT have made any difference at all.
One thing about the Speed Density EFI systems, they are NOT tunable unless you have some programming tool (like the Hypertech). I don't think there was anything like that in '89 for Camaro's. The Speed Density EFI system, at Wide Open Throttle (WOT or Max Power) the A/F ratio is not the stoichometeric 14.7:1, thus the typical switching oxygen sensor is worthless for feedback. So at WOT, its just dumping a predetermined amount of fuel into the motor based off testing of the engine as what should give the desired A/F ratio, but the PCM has no idea if its the right A/F ratio or NOT, and the PCM can't use the O2 sensor to tell, so it is totally blind if it has the right A/F ratio or NOT. The manufacturer, to prevent engine damage, goes to the safer side and dumps more fuel than it should to give a richer A/F ratio. (Too lean at WOT causes exhaust temps to spike and burns valves, Cat's, does all sort of damage). So, its a safe bet that leaning out the A/F ratio at WOT throttle will bring it closer to the best A/F for WOT and produce more power. Since the amount of fuel being dumped into the motor is static, increasing the air flow through the motor at WOT will result in leaning out the A/F ratio, i.e. more power. And that is why sometimes, some engine/platform combinations that might have a single choke point in the air flow through the motor, using a speed density EFI system, if you free up that choke point, it could make a big jump in power. While other platforms, using the exact same engine, might NOT see one bit of power change swapping with the same part.