Adrian Flux came out best when I wanted to insure my 200tdi 90 Defender, from memory around £180 but your right a galvanised chassis will increase the overall top end valuation which I believe you can state when making your declarations at the time when arranging the insurance policy and also declare any modifications from standard that have been carried out, however, is the replacement of a standard chassis to standard specification a modification, I don't think this would be classed as such but it's certainly an enhancement, good luck in any case.
Jock