I also agree you should stay with the smaller carb until you decide to upgrade for more power. Advice by CHAS to go with AFR 185's is probably the most valuable so far as the heads will be a major power making component . Since you have objectives now you may consider ordering the AFR's already milled to 58cc's .That would give you another .75 points compression over the stock 64cc's. For 500HP you'd probably want 10-10:5:1 compression, later considering the V-Jr intake port matched to your AFR heads, getting the larger carb, then select a matching cam that's a more aggressive profile than the B303. You can consult
www.bennettracing.com for a Comp cam custom grind that would get you where you want to be with your components peaking HP around 6K.