/6 or v8 swap?

The only way I would run a slant six is with a manual transmission.

I have a daily driver 68 dart and it started with an auto. Felt like it was hamster powered. I switched to an od 833 and what a difference!

Exhaust is the single most effective way to wake up a six. People usually put bigger carbs but that is useless if it can't expel the extra volume because an engine is an air pump.

A six will cost more to make a good runner unless you already have parts like headers or a improved exhaust manifold like one from a super six.

A 318 is reliable but everyone has one. A six can be made to go. I usually do 2700 RPMs at 85 mph on the highway with decent throttle response, even better throttle response a gear down in 1 to 1.

Throttle response seemed to lag more with an auto, so I really do believe the transmission is the key to a slant.

A 318 will run better with an auto. IMO although I have had 318/4 speeds before......fun !!!

If your wife is short like me then a piece of wood 2x4 duct taped to the clutch pedal will help. Definitely get a 3 finger clutch, it's worth an extra effort on the knee to get a better, harder shift.