Daveo makes some good points there.. There's no need to go as far as the bullet roadster with the space frame etc.. Looking at pics I think a lot of the custom work in the front end it is beneficial to suspension geometry, weight etc, but is also needed to accommodate the dimensions of the V8??
Seam welding with a comprehensive cage with under body bracing should be enough to keep the little MX-5 from falling apart.. After Brakes, suspension, LSD, seat wheel etc 20k is probably a more realistic budget.