I'm looking into replacing the 5 leaf van springs in my mk1 to a new set of king springs, it is a street car but may see the track in the near future. They will have to be modified to fit as mk1 leafs are 50mm and the kings are 60mm. Should I modify them for extra rebound leaf as well or just install them as supplied?

You can buy the 50mm springs for the mk1 through kings... just ask them...
 
 

Ok cool, should I install them as they come or get an extra rebound leaf added

 
And you can fit the MK2 width leafs into a MK1 without modifying the spring.  You need to machine down the front bushes and use MK2 rear shackles and spring plates under the U bolts.
 
Not very hard

So if you have a 1975 mk1 shell what would you order? could you get the mk2 springs and they fit with no mods?

Rang king springs today and they dont do the mk1 springs anymore. They would have to custom make them which is very pricey. Better off with modifying the mk2 FOR-024 springs to fit

Took some measurements of front spring mounts and they are 65mm ID. Wilkinsons suspension have advised me to not machine bush down as there will not be enough material to fit and withstand sideways pressure for diff movement. They grind down the front eye of the spring to 50mm and modify bushes and crush tube to fit. They have apparently done it many times before.

 
Thats crap!  My Mk1 and a few others run like this and they are predominantly track cars!

Kizzy

Took some measurements of front spring mounts and they are 65mm ID. Wilkinsons suspension have advised me to not machine bush down as there will not be enough material to fit and withstand sideways pressure for diff movement. They grind down the front eye of the spring to 50mm and modify bushes and crush tube to fit. They have apparently done it many times before.

 
Mine and at least 3 others I have done all run the wider springs with a modded front bush.  I looked at grinding the spring down, however IMO modifying spring steel is a no no, and creates a possible problem area for strength.  The statement about there not being enough material for sideways movement is complete crap, even with the modded bush there is plenty.
 
Woz

I would have thought that you'd get most of the movement from the rear spring hangers anyway rather than the front ones.
 
Matt