Just like to share this example from the other side spectrum of racing traditions with retro closed coupes. Here's a beautiful example of taking a Fiat Simca (very similar body style w/ your Topolino) with great lines for great 'top speed' performance. It is a closed body example instead of your open rail design, but it has spun wheel discs (decals!) & interesting rear wheel spoiler fences & diffusers. Member of the 300 mph club & 'Baddest Race Car' in Bonneville Speed Week 2010!
There are also other 'Top' dragsters, and especially the famous,Top Cop, racing dragster with its front grill engine fairing and rear angled top wing. I don't know your rule book for your racing niche but if you could add some/all of the aero advantages you are 'allowed', we're sure you could earn a few hundredths.
Bonneville Salt Flats Baddest Race Cars - Fiat Simca | MyRideisMe.com
Fiat Toppolino's - Photos anyone??? - Page 8 - THE H.A.M.B.