Craig,
I finally gave up on Ians springs also. Now running Kings 450lb front and 500lb rear. Theses are not much different to Ians rates but they are the rate they say they are unlike Ians Blue springs which are all over the place.
I went with 250 mm long springs but expect these to be just a bit too long and I may have to change to 200mm long with keeper springs.
The king springs are a larger id (and OD)than Ians so you will need to make up a spacer for the bottom adjuster. The springs will fit easily on the rear but you may have clearance problems on the front depending on what top arms you have. (some are straight and some curve in).
Looks like you are getting really serious about the Nationals. If you are looking for a tuner that will out do any one else for peak and average under curve power use Sonny at Marranos 9744 5355.I reckon your combo will be good for 420rwkw
Phil