67 Valiant power steering way too touchy

The trouble is not with the steering pump "putting out too much pressure" -- that is not how it works -- and there is no special/different pump check valve for V8 vs. slant-6 pumps because there is no such thing as V8 vs. slant-6 pumps; the difference between slant-6 and V8 is in the bracket, not the pump. The '60s-'70s Mopar power steering systems have very little road feel or self-recentre action and can feel twitchy/squirelly. Alignment is critical; you want as much positive caster as you can get -- offset bushings and adjustable strut rods help here. Take a look at the FSM; the alignment specs for the nonpower-steering setup call for neutral to negative caster because otherwise the non-assisted steering wheel is too hard to turn at low speeds as when parking. The power-steering specs are for positive caster to bring some road feel and self-centring to the system. Moreover, the alignment must be done all the way correctly, which means setting the ride height as the first step. Very few techs at very few shops know how to check and adjust the alignment correctly on a car such as ours. Finding such a tech at such a shop is a worthwhile pain. Also you will get a sense of "twitchiness" if there is play in the wheel caused by the steering gearbox being out of adjustment. There are two adjustments; see here. Once all of this is checked and you've also fixed any other slop in the system (Bushings, arms, links, ball joints...), then it's time to look at a steering gearbox built with stiffer reaction springs. These can be had from Firm Feel or Steer & Gear.