As far as I know the slant six k member is as strong as the V8. For added strength many people like to fully weld all the seams on the k member. It's not required but it's an easy upgrade if you have a welder.
If you are using this strictly for drag racing many people leave the stock slant six torsion bars in the car. For driving on the street I think you will like V8 torsion bars much better. The difference between torsion bars is the diameter with the slant six bars being a smaller diameter than the V8 bars. With that said there are several choices for V8 torsion bars, I should also mention that there are different lengths for different body styles so one needs to keep that in mind when looking for them.
