I have a 73 Road Runner that came with a 400 engine and an automatic. My question is: Are the 400’s from 1973 internally or externally balanced?

I put in a 451 engine that is internally balanced and uses an ATI damper. I reused the factory torque converter and flexplate. I have a lot of vibration and am hunting for what it might be.

the original 400 would have been ext. balanced, and the stock TQ will be wrong for your Int. balanced 451

Knock the two weights off your TC and you'll be good to go.

My next question was going to be is it the flex plate or the TQ that is not neutrally balanced. I think you just answered that one for me.

Yep, take a grinder and zing the balance weights off the converter and you'll be good to go.

I was wondering what people here would suggest for getting them off. I took a look this evening and they appear to be held by two studs or maybe they are very precise spot welds. I attempted to drive a 3/8 chisel between the weight and the converter but had no success at all. They are about ¼” thick. That would be a lot of grinding. Other suggestions?

Are there only the two next to each other or are they spread out all over the TQ where I haven’t seen them yet?

Having done this several times using different methods, this one worked out best:

Grind a slot through the middle of the weight down into the spot weld.
The slot's purpose is to remove material from the middle so that ea. end of the weight can be pried up and "snapped" off with a cold chisel.
Then grind smooth.
There should be two weights. On on each side of the drain plug