Jonathon, I'm getting a bit confused here. Do I take it that the current rack on your is not a R4 one, but you want a R4 rack because it's better spec than the one you've got??
Is fitting a R4 rack to Manugusta's a common modification? as I looked on a De Tomaso website yesterday and saw a R4 rack fitted to a Mangusta. The rack I saw was the earlier type.
In that case if all R4 racks attach to the chassis in the same way then maybe a later rack will fit. Malcolm may have the answer to this one.
Steve