Keep in mind that these cars weren't designed with headers in mind, so no matter what header you install you're going to run into a few issues.
I have a set of the LPPs on my 383.
There is no problem with the O2 being directly on top as it is located forward of the floorboards.
Some folks have complained about steering column clearance. This is an EASY fix. Simply loosen the three bolts holding the column to the firewall and move the column so it clears. I had to do some VERY MINOR grinding of the steering shaft to clear them, but other than that it's a very minor problem.
You will need to delete the low oil level sensor as well as there is no room to plug the harness connector in. I opted to leave the sensor in there and just leave it unplugged. Remove the bulb from the dash and you're set.
Overall, plug access is good, and the gains are great. For the price, they are legit.
I've been using PCMforless.com for 10 years now with zero complaints.
Good luck.