The good news is I have finally got a pair of manifolds that will fit with some minor bulkhead chopping...
My problem may not be a problem based on the replies to this...
The left side is taken from a Stag / RV8 the other is a Landrover Discovery.
The primaries are all the same diameter its just the outlet size that differ so I was going to cut the small one off higher up so it then matched the larger bore size and then have a same size bore pipe welded on.. makes sense right ?

That way all the pipes are the same size and should cause no imbalance in flow between the heads... it will be a road going car, no fancy engine stuff just a standard 3.5 V8..
Any thoughts ?
Stu