just a thought (possibly a stupid one)

[Lukewarm Wilson]

Has anyone looked into putting the gs400e head onto the GS500 I've noticed that the 400E is supposed to be a 4 valve head and is still from the basic GS twin motor so may possibly just bolt on. With a little port and valve job it maybe a good thing a bit easier then say a GSXR head but not as sexy though and probably not so workable, but may make a good street modification.
Just a thought anyway  

[Blueknyt]

GSX400E  there is a GS400 aswell  the X designates the 4 valve head.

no reason you couldnt do it with the early 80 GSX750 heads that were aircooled before the GSXR came out.  fact, if you find an 1100E engine you can use those pistons too