RE: UK Magnums
The magnum R is the second generation. But I nnever got one, I dont know if the R ever made it to America.
One layer of wing sadle foam tape should fill the gap.
Nelson 40 is a great choice. There was a Magnum-Nelson at my home field and it kicked butt.
It is best to beef up the firewall and wing mounts when intalling any high performance motor.