mornin'
you have to consider that teh P38 was nnever designed t obe run on coils so any coil conversion is not going to really suit the handling.
Ideally you should really put the air suspension back in but that's not going top be cheap.
the problem is that the colis aren't progressive so they are in effect too soft, especially when towing the 'van so your options are really to try to stiffen up the suspension either by thicker anti roll bars, stiffer springs, aditional shockers (I assume yours are in good nick).
Of course downside to this is that when you aren't towing it will feel really stiff and probably a bit "crashy", particularily considering our crap roads.
Other alternative is to keep to 40mph when towing but then you really aren't getting teh best from the P38.
Sorry I can't be more positive but maybe someone else with a coil converted P38 will jump in. Have you checked on Rangerovers.net as a lot of the yanks put there's on colis but I don't know how many of them tow
