What's the difference with a 1970 HP440 bottom end that it needs the 1970 "six pack" balancer?

Is it just the rods that are different?

or is crank different also? Are All 440 Steel Cranks balanced the same?

If after market rods (eagle H beam) are used with a 70 HP STEEL Crank, which balancer should be used?

The "six pack" rods are heavier so those motors are externally balanced. They use the special weighted balancer and a weighted torque converter.
If you change rods, then the assembly needs rebalancing and ideally you'd change it to internal balance at that time. Use a neutral balancer and converter. You can remove the weights from the stock converter.
Internally balancing makes everything easier...no worrying about weighted converters or special scallop cut flexplates.

Unless you want the 100% correct resto look, internal balance is the way to go.

I'd recommend you read through this.

Dampers

see what an8secCuda says.

The forged 440 crankshafts are all the same, except "I think" the six packs got a .001" surface hardening (which will be gone when polished or turned, so don't worry about it). The rods are heavier, that's it. The dampener & flywheel/torque converter weights balance out the heavier rods.

For your '70 steel crank, I suggest you buy a new, neutral balanced harmonic balancer (dampener) and neutral balance the engine with the eagle rods. Also, neutral balance your torque converter or flywheel/clutch (separately) too. This makes changing out those parts in the future MUCH easier.

Chrysler tufftrided (chemically hardened) all the Mopar cranks starting way back, they should have nitride them

The rod and piston weight require the external balance, the lighter '72-later piston didn't need external balance with the heavy rods.