Kris you must match the balancer to the crank period. The stock "400" requires an "unbalanced or externally balanced" harmonic balancer (I am assuming you are doing a 0.030" over to create your 406). If you use an aftermarket crank some are externally balanced (most) and some are internally balanced, so it is absolutely imperative to make sure you use the right one. To further complicate this the "flexplate/flywheel" is also an "externally balanced" as well on the stock crank. Aftermarket would again require research. I have never seen a solid front Balancer as that would be a "flywheel" and would not dampen any crankshaft harmonics the sole purpose for it being there. As for your transmission choice just make sure that the correct flexplate has the right bolt pattern for your convertor, just to separate the boys from the men, (as it wasn't hard enough to start I guess). I know that this has made more questions than answers for you in the short haul but this info will be fairly easy to get on the net if you are patient, and should be easy if you are using a stock crank, just match the year and displacement, as the 400 is a unique beast. Please keep us posted and I am supposed to bug all new guys for pic's! And that means I am preaching do as I say, not as I do!
Harry
I'm not a hoarder I'm a preservationist 78 Monza Spyder (~Soon(ish +/- I guess) To Be 2+2 with Spoilers)