After experience with Lucky, I can say that none of the downdraught carbs are any better than properly sorted SU's. Spend the money on a decent filter system and getting the SU's set up on a rolling road. Nrver assume that the manufacturer got it right and that the only way forward is to change components!
When I were a lad, and there was no such thing as fuel injection, the world divided into two camps - the SU and similar camp and the Weber and similar camp. Weber's were broadly supported by the racing brigade, who were only interested in full throttle and full revs. SU's by those who drove their cars in ordinary traffic. Webers win at full throttle and full revs because there is nothing to disturb the air flow, so they generally shift more air. They can be made to work in normal use - but it is a work of genius to set them up. SU's are vastly easier. This is because Weber type carbs rely on all sorts of supplementary widgets - extra jets, accelerator pumps etc etc - to cope with part throttle running. The SU dispenses with all of these and manifold vacuum does most of the work. Downdraught carbs all belong in the Weber camp. nuff said,
Chris