Hi Simon,
I wish that I could offer you some wisdom here but I too are struggling with my setup and far from happy with it.
I do have some questions though;
You mentioned that you started fairly softly sprung - what sort of rates were you initially using at a point where you were relatively happy with the car?
Rear 250 lb springs - how did you rate these? I have searched high and low here in Christchurch and there is no-one who can rate leaf springs ( including the spring makers Bellamy and East )
Any reason for not running a rear bar?
Also your front end- have you had any bump steer issues after moving the pivot points for the lower control arms higher in the crossmember?
Interestingly from what I have read and responses from various 'learned' people on this site 350lb fronts is the stiffest that I have seen on a 1200 with an A series. After saying that I have had suggested to me that my 275lb fronts may be too soft and certainly given some of the rates that people over here are using on say Pre 65 Ford Anglias ( 450-550 ) it would appear too light yet it does raise the question is it best to tie them up really tightly and turn them essentially into go-karts?
I'm sure that you will get it sorted and look forward to following your results next year.
Dave