But I'm still wondering why he'd pay Mario $$$$$$ to chop up two valve covers to make one when one of the chopped up covers being original would have been just fine, I mean talk about a waste of money, unless there something in the valve train requiring the S14 hump but I doubt it.
Cmac covered it quite well, but I will elaborate a bit more.
The S13 engines have a catch can on the side of the cylinder head which can get in the way of downpipes or other components on an engine swap. Nissan realized that they could get rid of that catch can on the later SR20-series engines and "humped" the rear of the valve cover to build in an internal baffle. Since the S14/S15 series engines have a variable valve timing, they have an additional kick out on the front of the valve cover to accommodate that on the front of the intake camshaft-- but that makes it so that valve cover won't bolt onto a S13 engine.
To get rid of the S13's side mounted catch can and take advantage of the better packaging that the S14/S15 valve cover offers, a "Hybrid" valve cover is made-- weld on the front of the S13 valve cover to the back half of the S14/S15 valve cover and you get the best of both worlds.
This link shows the process well: http://tougefactory.com/shop/2014/12/service-s13-4-sr-valve-cover/
We have done this conversion on S13 engines and it works great!
Hope this helps,