A 350 poncho has a small bore. 3.875 in
The heads with the larger valves create a flow problem as the valve is too close to the cylinder and "shrouds" the valve decresing flow.
350 have chamfers at the top of the cylinder to help unshroud the valve and inprove flow.
I wouldnt spend much money for heads for a 350.
If the motor is running , i wouldnt do anything but a cam and some carb work. And some dizzy tuning.
Even milling the heads that are on it would raise the cost as you would need to make the valve train adjustable.
I have a good running 350 in my poncho. Its coming out this summer for a 389 replacement.
I am even questioning that...now that I have a 428.
I will mothball the 350 for anything I get in the future that may need an engine to be a driver.