OK Bill, it seems that both of us are in the same place as far as forge building and smithing experience goes, so you know that what I have figured out is not based in hard experience. If it makes sense to you it may be of some assistance, use it as you see fit.
What works on one mans forge may not work on another. The number of variables in any forge set up ensures this.
Some of the variables, other than the blower itself :-
1. Diameter, length, shape (bends etc,) air tightness and materials of air delivery plumbing. All of these will have an effect on the percentage of loss in air volume, (cfm) and air pressure delivered to the tuyere. {Your lungs have a fixed capacity to exhale, if you blow that capacity through a drinking straw you can create a strong blast of air, if you do the same through 6 feet of vacuum cleaner hose you would barely notice the result at the other end.}
2. The tuyere design. How well the ash dump seals off the bottom of the tuyere and how much the grate (and fuel and clinker) restricts air flow out of the tuyere.
3. How big you need the 'hot' fire to be. (A fire big enough to heat 1/4" round stock would not be enough for 3".) The bigger the fire you need the more air you need delivered out of a bigger tuyere.
Whatever blower you use, hand crank, bellows or electric, it will need to have a capacity in excess of your forges requirement in order to overcome losses and restrictions within the system. The excess air is then 'tweeked' out by using intake restricter, inline air gate or air dump. Obviously if you go too big the ability to fine tune would be soooooo much harder to achieve.
From Mr Woolridges post in the thread above I am assuming that a blower with bigger blades that moves more air per revolution is going to work better for forging than one with little blades that is spinning like crazy. We seem to be more in need of air volume rather than high air pressure, but air volume that will not stall when confronted by restrictions in the system.
On that basis I would opt for the 128 cfm blower you spec'd earlier. It would not be too hard to glue, stick, weld or jam a round pipe into a square plate to make a manifold for the square outlet on that blower. Being that it is built with an intake restricter this should provide you with the all important 'tweekability'.
Oh and I had another 'WOW LOOKIT THAT' moment. To keep track of a thread just click on the bit that says 'Thread Tools' at the top of the page, then click subscribe to this thread. Sure beats the heck out of trying to remember and find the ones yor interested in.
Let me know how you get on.
Mick