Seam weld the shell, (if required and allowed in your regulations) making sure that whilst the car is strong it doesn't lose any crumple or energy dissipation zones.

Install Roll cages, Bolt-in cages if you're keeping to a budget or weld-in cages these can be pricey but will offer you much more safety and superior chassis rigidity.

Additional strengthening/gusseting 

Seam-sealer. Why? water ingress can very quickly cause rot in a bare metal shell, also the noise of un-seamed panels vibrating is deafening inside the car and can cause an increase in driver fatigue during longer races.

For extreme use cars (i.e. stage rally, off-road) additional underbody stone chip guard can be applied. 

The body shell can be painted underneath, inside and outside, in the colour of your choice and that means it will have the same paint finish underneath and inside as it does on the outside.

We also specialise in bodywork restoration of rally cars, sports cars and vintage vehicles, we can accurately reconstruct and paint body panels to their former glory where quite often these are perceived as irreparable.