The counterweight for their rear rod throw is still evenly distributed in the correct manner; with half of it in front of the journal, and half behind it. Unlike the 400 or similar motors, where there isn't enough volume to put the correct amount of counterweight on the inside of the journal (along the axis of the crank toward the center), and therefore too much has to go on the outside of the journal (along the axis away from the center), the 350 crank allows for the theoretically correct weight on either side of the journal, such that it is dynamically balanced. The 400 has to be done this way at BOTH ends. The reason is, that the piston is tall enough to support what the factory considered to be the minimum acceptable ring package, that the rod had to be shortened; and the bottom of the piston comes so close to the crank when it's at BDC, that there isn't enough room left for an adequate amount of counterweight. That's why, in order to "internally" balance a shrt-rod 400, you have to use metal that's MUCH denser than the material the crank is made out of.

"Internal" and "external" balance DOES NOT refer to which side of the oil seal the balance weight is on, or even whether it's part of the crankshaft proper or not.

Pre-86 SBCs had the rearward portion of the balance weight located on the crank flange, which has that funky counterweight-looking shape to it (imagine that). 86-up ones, since the seal requires that the flange be no larger than the seal, require that the rearward weight be moved to the flywheel instead of on the crankshaft, since you can't very well slide a round lip seal over that other shape.

So, the weight on the 86-up crank "looks" like it's "external", and it is in fact physically "external" to the casting; but in reality, the design is still "internally" balanced.

Which means, that all you get by asking this question, is a whole bunch of confusion. It's better to avoid those words altogether when discussing these motors, because even if you're technically correct, people will misunderstand you about 90% or more of the time. If you say those words to your machine shop or go looking for parts or whatever, you will get hosed. The best thing to call it, is "stock" balance. Which is technically "internal", but requires that part of the counterweight be "external" to the crank, i.e. on the flywheel.

Some of the cheaper 383 stroker cranks ( or stroker kits ) are based on a 400 crank casting which requires both a weighted flywheel and balancer