[CobraMarty]

Just thinking, how about a single turbo instead of the elusive Gavin or ESS supercharger.

I don't know it they would fit, but the options could be;
1) N55 stock turbo, manifold, and downpipe
2) HPF N54 single turbo manifold and smaller 300-350hp turbo
3) Vishnu N54 single turbo manifold and smaller 300-350hp turbo

You could use stock 335 intercoolers (cheap takeoffs) and some piping pieces.