Jbrancheck,
Your plan will work fine. As noted, the 261 came out in '54 and used that style of pump.
There really isn't any difference in the cooling systems of the 216/235/261 other than the pump and the siamesed block on the 261, which hasn't proved to be an issue from anyone that I've talked with that has used the adapter plate (or runs a '54 261).
The main reason for doing this is to get the fan back into the correct position in relation to the radiator on '54 and earlier vehicles.
I would try to keep the pulley diameters as the factory intended for each type of pump, as changing the speed of the pump too much in either direction can cause overheating due to either not enough flow or cavitation. Just how much you would need to change the diameters to bring this about, I couldn't say for sure. If somebody wants to find out, please keep us all informed of your results. Below a certain rpm (whatever that is??) it wouldn't be as big of an issue.
Changing the balancer shouldn't matter to the balance of a stock engine. However, if you plan on a crusing rpm higher than about 2600, I would strongly urge you to spend the extra $150 (or so) on getting the engine balanced. I've had them both ways, and I will never build another one without getting it done.
