340 vs 360

65s said
340 = internally balanced
360 = externally balanced

What does this mean exactly? I want to hook a 360 up to my '65 cable shift auto 904 tranny. It has the small snout torque converter. Right now it's hooked to a later 318, so I have a machined spacer ring to take up the space between the small snout on the TC and the larger hole in the back of the crank.

Would I have to weld weights on my TC? I've heard of a 360 flex plate? Are all 360's balanced the same, so would any balanced flex plate do the trick?

Internal; It means the whole rotating assembly is balanced as a single unit
the external is not . The balanceing weights are on the damper and flywheel/T-converter.
In general, a B&M or other compnay have a flex plate that is used for this swap. Look into it.
I'm not sure about the older push button trannys and any lil'weird thing it may have.