[blykins]

There is a higher clamping load 11.5"/12" pressure plate available from McLeod. Part # 360650. I emailed McLeod for the exact pressure, but my guess is that it's about 2600-2800lbs. It's $210.

With that particular pressure plate and the 500 series disc, the clutch rating is between 500 and 550hp.

If you baby this thing around and never get into the secondaries like you say, much less have it on the strip, I'd be tempted to tell you to reuse your existing plate with the new disc. Also, if you truly think it's just 450 hp, then again I would stay with the smaller plate.

I don't really know why the springs gave up, but if they're as old as you say they are, they could have just softened up. Valve springs tend to do that too.