Holley 650 dp jet sizes????

Can someone give me a starting point for my holley 650 dp carb?
My engine combo is 355 sbc, 9.5:1 cr, Vortec heads, Comp Cams Magnum 280H hyd. cam, 1 5/8 headers. It now has 70 in primarys and 80 in sec. I think that is too rich. This is a brand new engine with maybe 50 miles on it. I put Champion RS12YC plugs in it and they are beginning to foul out. I thought it may be the jetting, so I looked at the carb jets. Any thoughts???/

Try 65's in the primaries and 76's in the secondaries. I ran that combination in a 400sbc with an Isky 30/30 solid lifter cam with pretty good results

NOt sure but think that plug you have is too cold for your app....