'68 Frame stiffening?

'68 El Camino so the frame is already boxed from the factory.
Still seems pretty flexible to me.

Picking up another frame on Tuesday from a '69 El Camino.
Not going to be pulling the wheels off the ground racing but a stiff frame helps handling as the suspension works better.

Wondering what might be done without a crazy amount of fabrication or adding a full roll cage?

I have all the welding tools, and will have a bare frame to start from.

Also putting in an IRS, probably from a Jag since the 'Vette stuff is harder to get and more expensive.