[Geoff Kingston]

Unless you are very lucky the interchangeability of bonnets on the 401 and 403 is unlikely, as explained in the workshop manual the bonnet skin is cut and adjusted to fit the opening in the body.
I had the original bonnet for my 401 but the metal frame had long rusted away and been removed along with the bonnet catches.
We had a complete bonnet from another 401 but it would not shut because on one side the claw shaped catches would not clear the body to engage with the locks and even if they had the bonnet would still not shut as the skin overlapped the opening.
We decided to take the catches off the frame as they have to de adjusted both in position and height to make the bonnet fit properly,this takes a lot of time and needs to be done carefully due to the build up of rust on the threads, we then removed the steel frame off the bonnet reattached the catches and then fitted it to the car adjusting as necessary to make sure it worked properly.
The bonnet comprises a skin, an alloy sub frame and a steel frame, what we discovered is that the steel frame and the alloy sub frame were jig built, every rivet hole lined up and once the steel frame was secured to the original bonnet and trial fitted the result was very impressive.
If you don't have a bonnet its going to be hard work to fit a second hand replacement but if you have a bonnet with a rusty steel frame then a good second hand frame or bonnet with a good frame should do the job.
It took about a day to a day and a half to sort it all out but I think it was easier and cheaper than trying to make up all the missing parts and an interesting alternative to the procedure outlined in the workshop manual.