If you want to stay with 15", the M/T SR radials may be the best d.o.t. approved tire for your application. They are speed rated to H, have a modern tread design, and a fairly stiff sidewall. Since they offer these in wheel diameters from 15-20", I'd say they are a more modern tire than a BFG.

The Viper caliper kit will fit inside 15" wheels if you use the stock 11.75 rotors. Stepping up to 13" rotors will require the 17" wheels, however.

The guy telling you stories about 15 vs 17 inch tire deflection is doing just that; telling you stories. It is unfounded. Negative camber gain is the result of the physical dimensions and layout of the suspension parts and pick up points. It has nothing to do with tire sidewall. Sidewall height is a part of the total suspension make up, but will not change camber gain all by itself. This is one of the advantages of using the FJM spindles. Their taller height creates additioanl camber gain through their cycle because they shorten the effective length of the upper control arm.

BTW, if you want a really trick, cost conscious track only tire, check out the Towel City link above. Their racing slicks have a couple of degrees of camber built into them. For more grins, put your rear axle housing into a press and bend the tubes 1-1.5 degrees to create negtive camber in a solid axle rear suspension as well. That really messes with the IRS guys when you can lay down as much lateral grip as they can.

The thing about 15" tires being better for traction is somewhat true for drag racing applications where the large sidewall can absorb the launch shock and create grip instead of spinning. Not the greatest thing for lateral loading, however.

An intersting note is that to generate around 1 g of force on a drag strip launch, you would need a somewhat dedicated drag race suspension in a somewhat serious effort bracket racer that would not be much fun on the street. To generate that same 1 g of force laterally, you only need the right combination of components that will make your street car more fun to drive. Kind of makes me wonder why more guys aren't thinking handling instead of drat times.

For the leaf spring rate, it is all about the final wheel rate. Looking at their web site, the XV front t-bars and s-bars look to be around 1.125, smaller than the 1.22 your planning on using. Similarly, they use a pretty sizeable rear sway bar, looks about 1" in diameter. Another thing to note is the XV rear bar is much more straight than a stock mopar rear bar. So even though it is not a lot larger than a stock bar,.75 vs 1.0, the lack of bends create a more rigid bar that is going to more effectively control motion than the pretzel shaped stocker. So you are putting more wheel rate into the rear with the sway bar, so you can go lighter on the spring rate.