I,m rebuilding my Dell 40 carbs and wondered if its ok to use the card gaskets to mount the carbs to manifold as ive seen fitting kits available that use O,rings etc.
I assume this is to counteract vibration and to maintain a good seal.
Any oppinions/experience out there?

you need them to cut out the vibration other wise the fuel is full of bubbles

Do not use/make gaskets, for the reasons Macspeedy mentions. By far the best things to obtain are 'Misab' plates (rubber O rings cast onto an aluminium plate) and rubber bobbins/cup washers. Available from all the usual suspects (DT, Burtons etc).