If immediately before the 1st April 1974 there is an existing joint planning board constituted by an order under section 1 of the M1Town and Country Planning Act 1971 for a National Park comprised in two or more existing counties and as from that date the Park will be comprised in two or more new counties, the Secretary of State shall make an order reconstituting the existing board for discharging the functions to which this Part of this Schedule applies and F1section 2 of the Town and Country Planning Act 1990 shall apply to a joint board so reconstituted and the order reconstituting it as it applies to a joint board constituted under that section and the order constituting it and shall so apply as if the area of the Park were a united district.