I can definitely recommend Avon Customs, but the fact they do a very good thorough job means they are not necessarily the cheapest or quickest! they did a great job on my car though, which i can send pics of if you wish.
You could ask Martin for advice, he owns the garage in Whitchurch (near the piano shop). He has had Z31s, Z32 and at present has a 240Z with my old 280ZX engine in it!
He might be able to recommend someone (also we use him for servicing, MOTs etc for our Zeds)