I am considering replacing my stock valve covers with aluminum on my 340. I was told by my engine builder to get rid of my pcv valve and put a breather in it's place so there is one on each side of the engine. Not sure which ones I am going to get yet, but after looking around at pictures I noticed that all the aftermarket aluminum valve covers have the breathers in opposite corners (kiddie corener) from one another. Is there a reason for this (perfomance or mechanical related), or is it just the way most aftermarket valve covers come. I was considering having the breather cut outs cut directly in the center of the length of the cover. The factory covers have them in the center so I can't see a reason why it should be a problem. All advice and pics is appreciated.

most aftermarket are made on one side thats why they are oposite.custome ones have them in the middle like moon or offy's.just remember you need a hole for the oil fill.

I would keep the PCV. They provide good ventilation of the crankcase and help to prevent blowing out gaskets. Here are a couple shots on how I routed mine on my Duster with MP valve covers.
Drivers side PCV to carb.

Passenger side, factory breather to air cleaner.

If you don't want oil film all over your motor put a breather on the passenger valve cover and the pcv on the drivers side. Do not do away with the pcv.

PCV stands for POSITIVE crankcase ventilation. It is in essence an early and "factory" version of a crankcase vaccuum pump - which you will see on many race motors these days. Another "race" version is the pan-evac system whereby crankcase vapours are draw out at the header collectors. Sucking the vapours out is a good thing ... the PCV is a keeper.

Thanks for the advice. I would like to know more about eliminating the PCV valve my engine builder say it is not good to burn those fumes.