1959 235 6 cyl. tune up issues......

[john dob]

hi - i have a 1959 chevy apachie 3100 with a stock 235 6 cylinder and 3 speed on the column. the truck runs great up to 45-50 mph then it sputters like it is running out of gas. i have done the following to the truck: new gas tank, new fuel pump, new fuel filter, re-built carb (rochester type B), new plugs, new wires, new points, new condenser, new dist. cap, new rotar. the dwell reads right within in specs (approx. 30). i can only set the timing by ear and feel. i have looked a few times and can not find the timing ball mark on the flywheel. i even took out the plugs and rotated the motor by hand a few revolutions and still can't find the mark. i advanced the timing until it kicked back on the starter and backed it off a bit. starts, idles smooth and runs great until i wind out second gear and try to accelerate 3 rd. gear past 45-50 mph. i thought about changing the float setting but it is right where the instructions said it should be. i am thinking the timing is either still off or i have some carb. issues. hate to spend the $300 on a carb. and still have it act up. any thoughts would be greatly appriciated. thanks - john (wilmington, delaware)