Slant got me stumped

Just my two cents worth...

Timing on my slant went south one day acting pretty rude, the distributor hold down bracket under the distributor loosened up an the distributor turned some. This is not the bolt you see but underneath the distributor on the oil pump side.

Also the head gasket can be bad with no bubbles in the radiator. The gasket can blow between the cylinders or into an oil drain hole.

Honestly It sounds like a vacuum leak to me. Possible leaking from intake to head, but when you changed the intake that should had solved that. If brake booster hose or check valve or diaphragm was leaking that would do it also. Throttle cable sticking or kick down linkage hanging up??

In 73' they are kind of still very basic. After 25years of turning wrenches for a living ALWAYS go back to the basics.
Good luck!
Joe

Joe