make one change at a time. change the timing go up 2degs and then 1 deg at a time to see were it is. give as much timing intill the mph falls of and then pull it back a deg. most 400's that i have had on the dyno with about the same combo you have like about 35-37 deg of timing. lock out the timing i have and always lock out the dist on the street or track. if you pointer is dead on then you should see max power at or around this deg's of timing. double pumper carb of the same size that you have now or the 850 if it's a mech sec. the headers should be fine. if you want pull the header and make sure the header tube is the same size or bigger then the exh port and it will be fine. if the exh ports on the heads are bigger then the header tubs you have a problem.. if the car starts slow cranking because of the locked out dist then wire the ign box so you can crank the car and then flip the switch for ign and you should have no problems. good luck