if there's plenty of room then a cone cutter, (I attack it from both sides gently, to avoid stressing the chassis) or if space is very tight (might catch under chassis wiring etc) then I've reamed the hole out before now with the sanding cylinder thingie on a dremel (it wrecks the abrasive, so used as a last resort).
Watford Valves do a decent selection of valve bases if you'd rather just get the right one!
https://www.watfordvalves.com/products.asp?ID=6&man=83