I looked at the ISR website. Don't see LTH's for the 370 or the 350.
The best LTH's are FI, and PPE. Not a lot of results yet for the Motordyne LTH. so the jury is still out. For a shorty. Z1's are good. If you are going to install the LTH's yourself. The right should take about 1 to 2 hours depending on your skill sets. The left side will take you a minimum of 5 hours with a lot of cussing and hands that look like they have had been in a fight with a pissed off putty cat.
If a shop is doing it. Look for about 10 hours of labor.
There is nothing wrong with the FI cbe. Fit, finish and customer service is top shelf. The only problem is the wait. The people who told you to stay away from FI. Do they own a Z?
Sound is a personal taste.
I have FI LTH's, 12" resonators, and cbe. Along with the Stillen 3G long tube intake. And a few other mods. HP is 334 and 279 ftlbs.