Is it worth changing the chassis on a 90 td5 or replacing it
The back half of the chassis is rotten Or would it be better to change to galvanise chassis there is also some rot on bulk head so could do that at the same time.

I'd go galvanised.
Whether it is worth it or not is personal to you.

If you intend keeping the truck and have use of a second car, also deep pockets as you wont want to put rusty items back on the new chassis my vote is go for it...

I went for a new chassis and have spent about the same again on "bits and bobs" rebuilding. as swisstoe says you won't want to put rusty items back on to a new chassis.

I did also repaint it myself so this did add to the cost a little.

"I went for a new chassis and have spent about the same again on "bits and bobs" rebuilding. as swisstoe says you won't want to put rusty items back on to a new chassis."

If the vehicle is a keeper then do it, at least you know you won't be doing it again and if you were to sell and buy another vehicle, what is to say that won't be in a similar condition somewhere down the line?

I am about to be in this situation and am aiming to replace my 90 chassis next spring, it isn't anything like rotten and has so far gone 33-years without being welded but it is one of those jobs I want to do before it is necessary and while I am still up to it.
All my major components (engine / gearbox, suspension, axles, bulkhead, rear-tub etc) are either recent (new) replacements or re-furbished and will be swopped over in large lumps but it is all the ancillaries that I will replace while it is apart and it is these that can add to the cost (fuel-tank, wiring harness, brake-pipes etc). Then there are consumable items which are recent and have plenty of life left but make sense to replace while it is apart (exhaust, suspension bushes etc). Add all new fixings and a suitable chassis treatment (I am not a fan of leaving the chassis in bear galv) and the cost can rise.

Galv is the way to go with everything! If it is bad enough you have to ask the question then the answer is always going to be best to fit a new galv one. whether or not that stakes up financially and makes sense is something only you can answer.