My 67 was originally 6 cyl auto.
Now it's a beefed up 400 4-speed.
Don't know what diffy it is, but it's not posi.

When I put my foot in it, the car jumps erratically, mostly to the right. It's pretty hard to keep on the road. Front end is fine.
Any ideas what's causing the lurch ro the right?

It could be a drag in the right rear. Check your back brakes. While you are at it raise the rear end and rotate the wheels and feel if there are and catches or if it moves smoothly. You may have a rear end that's about ready to lock up.
Jim

Still running monoleafs and one traction bar?

Are your leaf springs secure? Check for three bolts mounting each of the front brackets to the floor. If they're loose or the clips are broken, the brackets can slide causing the effect you described.

Still running monoleafs and one traction bar.
I'll check mountings and wheel tonight.
Thanks for the tips; more later.

Also check the eye bushings on each end of the spring. You might need to upgrade to dual bars or multileaf rear to handle the extra torque. I'm sure you'll get plenty of useful advice as this is specific to the '67s.

Mono-leafs with a 400 4-speed? I can't see that being a good mixture. I twist my 4-leaf springs when I punch the bird, so I am wondering what happens to just mono-leafs with exessive torque applied.