if holley can't solve your problem you probably could use this proform main metering block:
http://www.proformparts.com/mistore/prod...mpany_id=100719

and sold at
http://store.summitracing.com/partdetail.asp?part=PRO-67166C&view=32&N=700+

'70 Duster 470hp 340/T56 Magnum/8 3/4 3.23 Sure-Grip