Pretty sure the Indybrocks come with a 2.05 valve. When you upgraded the valves did you stick with that size or jump up to a 2.08 valve. When I see a head stall like that on a flowbench and start backing up its usually a high airspeed issue. I'll bet its well over 400fps over the short-turn. With your 712 lift cam you are well into the affected area. The flow numbers you posted are in the area of what a ported set of heads with a 2.02 valve usually end up with. A 5200-5500 stall convertor would really help out at that weight (3400). I don't know how you guys make a small-block Duster weigh that much.