Do not go larger than 1.940" on the intake valves. If possible, upgrade to the 1.600" exhaust valves. If this is cost prohibitive, the smallest exhaust valve should be a 1.500".
This is assuming a great deal, to be sure, but in older 350 engines (4.000" bore, 3.480" stroke variety), going to a larger than 1.940" intake valve actually hurts airflow because the valve opens too close to the cylinder wall, which impedes airflow. Also, unless some porting work is done (i.e. grinding the intake port in the cylinder head), a 2.020" or 2.050" intake valve will slow down the intake charge (YES, SLOW DOWN!), the exact opposite effect you're looking for when building torque.
The reason we can go to a larger exhaust valve is the exhaust port is quite inefficient as stock; enlarging the exhaust valve actually opens and straightens the exhaust port, making the exhaust escape more easily. Having spent gases pollute your fresh intake is not the way to build torque.
Further to that, to increase torque, you will want to install a freer-flowing intake and exhaust system (headers with true dual exhaust, aluminum dual-plane intake manifold), although it sounds like you're just doing a quick top end fix at this point.
Other things to increase torque: small chamber cylinder heads with flat top pistons, good ignition system (MSD, etc.), spread bore carburetor (Quadrajet for example), different camshaft (higher lift, only moderately longer duration), increase connecting rod length, increase stroke, possibly change torque convertor, even change rear end gears.
thanks for the response guys.
This will be a complete rebuild off a 4 bolt main 350 block.
I thought about a diesel but the cost seems prohibative.
I have a set of early 80's 305 heads with the small chambers and tiny valves. Do you know if there is enough meat in these to increase the vavle size (i think they are 1 9/16int & 1.5ex). If not what is a good head canidate for a rework.
I'll be running a quadrajet and a low end cam, dual exhaust off stock manifolds