There's plenty of people offering galvanised chassis for Lotus Elans, Land Rovers etc. so I can't see that twisting should be a problem for you. Two things you will find with the galvanisers is that any completely boxed sections will have to be drilled to let the molten zinc in and out. The galvanisers will probably do this for you (they know where to drill for best results) and will charge what seems a fair bit for just a few holes! The other thing is the Zinc surcharge. You get charged a percentage on top of the price according to the current market price of the Zinc they use.
That's unless things have changed since I had galvanising done last Century!
Great chassis though (and great shed too), so well worth making it last a lifetime.
Good luck with the build.
Len.