My take:

SFT is right, if you are talking long tube headers they make a noticeable difference all over, and not just at high rpm. If you are talking shorty headers I don't think they get you much. By 1989 the LA motors were both wearing the same exhaust manifolds and switching to dual exhaust or even one 3" pipe would help.

The 318 roller cam is smaller than the 360 roller. Sometimes. It seems that Chrysler was really playing around with the camshafts. I used a stock replacement 360 roller cam when I did my '86. Best data I had at the time was the '86 318 cam was 191/191 and 0.390/0.396, while the 360 cam was around 201/199 and 0.401/0.399, something like that.
One more thing you can do is regrind the 318 hydraulic roller, doesn't cost an arm and leg, less than $150 from at least two vendors. The stock lifters just go back on. SFT would tell you to disassemble the lifters and clean any crap out of the inside. I didn't do that, but if you decide to do that don't mix up the little pistons from the lifters as they are precision fit. Also when reusing the lifters I try to put them back into the same holes. There was an oiling hole halfway down the body on my lifters, take note which way they are pointing as you take out the lifter. I was never sure if there was a preferred direction, and maybe it doesn't matter at all.

Good luck in your project! we are here with more help when you need it.

R.