Blimey, just following some of the discussion on Disco 3 forum re the rustproofing people Raj is booked with, doesn't make good reading or viewing the large and obviously already rusty places not treated in some pics posted there... mind you covering existing rust in semi hardening films isn't the last word in rustproofing in my opinion.
Raj mate, preparation is everything, unless you remove and treat the rust itself its no good at all covering it with similar treatments, all that does is covers up further deterioration and once water/salt get under the coating it can do its worst unseen till it eventually flakes off and the true horror is revealed.
There is an alternative, well several actually as this and other forums discuss, either someone needs to get to work on the rust then treat/paint/coat or you remove the loose rust and cover the whole chassis inside and out (plus exposed body areas such as inner sills) in oils or grease or one of the oil based products mentioned on this very forum, in some ways this latter method has its merits because you take steps to clean the undersides and recoat it every year, if you don't want to do this yourself it's something that can usually be done quite cheaply locally.
Look under any vehicle that has severe oil leaks, where that oil lands no rust forms, but oil/grease/similar is not a fit and forget method, it needs preferably annual recoats...the down side to oil/grease is that dust and muck sticks to it.
That motor of yours is a beaut, i'd want it cleaning off first of all (local commercial lorry garage should be able to handle that easily, but you'd be surprised what a good underbody hosing down will shift after you've driven in the rain for a day or two), then my suggestion would be to get the sort of job done on it that Dave_S is having done, stage by stage rust removal treatment paint then final coating.
With your Cruiser its only really the chassis various suspension parts rear axle and fuel tank protector that are rusting and none of them that bad, yet...to me it would be a crime to just cover that rust allowing it to get ever worse, it wants dealing with properly in my humble.
Hope you don't mind the input Raj, don't want to see you spending the thick end of a bag o sand and not getting the best possible job done.