I agree 100% on the bump steer. Designing for some "acceptable" level of bump steer is like your teenage daughter being a little bit pregnant. The Unisteer is a great product, but you can do a Vega box cross steer cheaper, and eliminate bumpsteer that way, too.

On your 4 lug/5 lug issue, lots of the speedsters in the mid to late teens had an option for TWO spares in that rear mount position. You could then carry one front, one rear and cover the bases. I still like the idea of having five lug all around, but it's not at all critical. With the size of the wheels & tires you're considering two spares might have you saying, "Does the butt look too big?"