[Kenth]

Balancing:
Pontiac flywheels and flex plates have an asymmetrical bolt pattern so that the flywheel will bolt on the crank only in one position.
There is a slight external unbalance built into factory parts to aid in the internal balance of the engine.
This amounts to about 6 inch-ounces which if not compensated for will show up as a pronounced vibration over 2500 rpm.

Harmonic Balancers:
The harmonie balancer is designed to dampen torsional vibrations in the
crankshaft.
Without it the crankshaft may break.
The harmonie balancer is not designed to compensate for engine balance and can only affect it if the balancer itself is out of balance.
Pontiac does not use any of the ' 'unbalanced balancers" like the 454 Chevy units, so Pontiac balancers may be interchanged freely.
It is critical that the balancer you select be in good condition; the rubber should be free of cracks and evenly spaced all the way around.
Normally, the heavier the balancer is, the higher the engine can be wound without the crank failing from the torsional vibrations which increase greatly with high speeds and heavy loads .
The rule of thumb is to use the most massive balancer you have available.
The best unit is the early Super Duty model, but requires the use of the early four bolt water pump and timing cover.
The stock balancer is adequate for all but the most severe use.
All 1968-76 harmonic balancers are 6.75" diameter and virtually the same.
The Ram Air IV balancer is the same as the stock harmonic balancer except
for closer tolerance on the balance.
Interchange problems only arise when using the 1968+ balancer with earlier timing covers, water pumps and power accessories.
However, all 1969-76 timing covers are identical so you will bypass any alignment problems by staying with the late model parts.
Beware of aftermarket timing tapes designed for the 1967 and earlier balancers which were 5.25" diameter.
Timing the engine with one of these tapes on the late balancer will result in a
retarded setting.
With the 1968+ balancer, for instance, 360 total advance would be 2 1/8" clockwise from the zero degree pointer.

[mgarblik]

Post #2 is loaded with good information for you to digest. Based on the information provided, this engine will be getting a flex plate, not a flywheel. So IMO, It really doesn't matter which way you go for balance. (full internal vs weighted flex plate). I prefer an internal balance as that gives you full flexibility for the engine in the future. (future use as a stick shift engine with aftermarket flywheel and so forth). Also, your desire to avoid adding heavy metal to the crankshaft may be flawed. That decision may or may not matter what flex plate you use because weight may need to be added to the front of the crankshaft as well which has no compensation on the front. Since a re-balance is in the plan, have it spun up and see where you land going for an internal balance. Honestly, if a little weight needs to be added, the cost is about the same as messing with a weighted flexplate. Almost every one I have used needed some weight removed or a little more welded on to get the assembly within 3 grams which is our default for a street engine. Hope this helps.

[TXSTARFIRE]

Originally Posted by Kenth

With the 1968+ balancer, for instance, 360 total advance would be 2 1/8" clockwise from the zero degree pointer.

Is that supposed to say 36 degrees total advance?
Thanks for the great info!

[Kenth]

Originally Posted by TXSTARFIRE

Is that supposed to say 36 degrees total advance?
Thanks for the great info!

Yes, got a zero to much there.
You can take the circumference divided by 10 to get 36° of the total circumference which is 360°.
F.e. you can also divide 360° by 12 and have 30° if needed.
Circumference of 6.75" is 21.195" divided by 12 is 1.75" clockwise from the original mark on damper.