I agree with all of the above. I too have done considerable homework before my purchase. Over the last two years I have come to see several different manufacturer cars up close: Shell Valley (My car), Everett Morrison, ERA, Factory 5, Unique, SPF. As stated above there are many variances between these few kit maker’s parts. Safe to say that you would be opening the proverbial can of worms if you opted for a mixed bag kit. You'll be forever looking for parts, and modifying others.
Your best bet is to settle on a kit that fits your budget, has the options, fit and finish you can afford and stick with that manufacturer.
Short of your engine and transmission there are a few components that can be interchanged.
Check out
www.cobraaccessories.com for an idea of some “swappable” parts.