The stock manual steering is a 3/4"-36 spline and would use a Borgeson part #013464, power steering is a 5/8-36 spline and would use a Borgeson part #0125164
Generally if the pump was the problem you would feel it in both directions as the pump does not know which way you are turning. This could be either a steering box or if a 4WD truck take a look at the universal joint on that side, if they are binding up it can create issues like this.
The stock shaft will work, you need to trim it and then drill and pin the included coupler to the end of your column shaft, we supply the drill bit and pins with the coupler.
If you are using our hoses make sure that the end with the longer metal hardline is at the steering box connection. Both ends of the pressure line have the same thread however the end with the long metal hardline has a longer thread needed to reach the flare inside the hose port of the gearbox...
Our box / coupler does not have the same spline as factory, Factory power boxes are a 5/8"-36 spline, factory manual boxes are a 3/4"-36 spline and our power steering boxes part #'s 800126 & 800127 have an 11/16"-36 spline. The 036425 coupler will not fit the stock boxes.
This is not an OEM gearbox, cores are hard to come by for rebuild so many of the "parts store" brands that traditionally have offered re-manufactured gearboxes have gone to offshore sourcing (China), this is a fairly new import and time will tell if they nailed it or failed it.
Quick thing you can look at to verify if you have a Saginaw pump would be the pressure hose connection. If it is on the back of the pump it will be a Saginaw pump. The 925113 has the correct hose fittings for our box to a Saginaw pump but we have not done any first hand fitment with the slant 6...
He only needed spacers due to it being a small block pump/pulley, the big block Mopar pulley has a different offset that pushes the pulley further away from the pump. Give our office a call you can send that whole pump/pulley in and we can swap it for you.