1971 Plymouth Scamp Acceleration Stumble

Hi,
I have a 1971 Scamp that has a stumble while accelerating and sometimes
stalls out. In neutral it can rev all day long, but under load it struggles.
Sometimes when its babied, it will accelerate up to 50MPH ok but it
struggles. fuel tank is fairly new, new lines, new filter, new wires, new plugs,
timing was way out and after adjustment it improved, valve lash was out and
after I looked at it it improved a bit more.

Weird thing is when I replaced the filter the car accelerated very well for
about 1/2 a block before stumbling again.

Any ideas?

Thanks