Your apparent problems don't make it an easy answer. Not quite sure how you manage to spin it up with a slipping clutch surely it would slip before spinning?? But here's my input (for what it's worth).
I doubt very much you are too stiff at the rear and in reality it shouldn't provoke spinning up. I have 500lbf/in springs at the rear and think they're about right (though further testing is required to be certain). You are a little under tyred but more likely they were cold and it was a little damp I suspect. The drag tyres I used for my 10.92 quarter mile were only 265. It does sound like you have a fair bit of lag and the set up means the boost comes in with a bit of a bang. I have a similar set up and if I don't use the nitrous the boost hits hard. I suspect the combination of damp conditions, cold tyres, and late boost has caused the problem.
I can probably lend you a set of rear wheels to try out. I will be taking my race car out testing if not this month certainly early January. Not sure where as yet but you are welcome to come along to discuss with Eurospec they will be able to advise better.