'67 hardtop automatic 400 with mild mod's (maybe 375-400 hp), going from an open rear to 3.55 posi this winter.

Right now I have the original single leaf's with the right side traction bar only.

Am I better to go with new single leaf's and add the left side traction bar, or convert to multi-leaf and no traction bars? Are the traction bars really not necessary with the multi leaf springs for this kind of hp with a posi? Or multi leaf with the single traction bar I already have if it still fits?

In '68 the multi-leaf spring most likely helped with axle windup, but not as much as the staggered shocks did.

Since a staggered shock upgrade for a '67 is pretty tough, I suggest what I did - and added the factory LH traction bar. Provides much improved wheel hop resistance while looking OEM. The spring changeover will provide less improvement as far as wheel hop goes.

In other words, the dual traction bars will provide the most bang for the buck.

Agreed on the extra traction bar. My 428loves the dual traction bars and posi unit...

I have no wheel hop. now traction is a big problem but I can live with the tire smoke!

Somebody put a 69 rear with multileafs in my 67 bird, no traction bars and non-staggered shocks. I don't have any wheel hop issues.

I have a 69 with multileafs and still run traction bars. Always have except one trip to the track when I had removed them thinking that maybe I didn't need them anymore. When straight home and put them back on before I took it out again.

I would say add the second bar first and then it you need more make sure your shocks are in good shape and then do springs if you still feel you need them.

All depends on how you drive it.

Mono's for max traction/racing.
Multi for better ride and cornering.

I see CI sells a traction bar floor plate. Since the car didn't originally have a LH side bar, I would need something up front for mounting. Anybody use this plate or fab one of their own? Is it welded or bolted in? CI doesn't specify if this if for RH or LH but I think they would be different..?
Thanks

Yes, they are different. I got a LH side plate from one of the other members here. It was a tad too rusty, so I had my favorite welder (and custom motorcycle frame builder) use the LH plate as a template and he fabbed up one for me.

Since I do not have a welder, I just "glued" my fabbed LH plate the the rear floor pans with panel adhesive.