In my quest for better handling, whats next?

I think the '68 Steering box mounting is probably better than the later ones. I think the OP's first welding mod - especially if autocross on R comp tires - ought to be subframe connectors. Keep in mind that Street Prepared and Street Modified classes allow welding of the frame connectors only at the front and rear attaching points. One additional non-welded attachment is allowed. Cross bracing and joining of the subframe connectors is prohibited. I mention the class rules because its something that bit me when I first started autocrossing. At that time, welded frame connectors put my car into Prepared class and of course I had already done it.