Hello Morlan
If i understand your son broke the throtle chamber flange.
If it hapened you can always try to get the throtle flange and assemble it on the carb. It will be less expensive than the US$300.
However, i think that it will be still quite expensive, since over here it costs something like US$150 (new).
I don't know how it works over there, but here in Portugal you can always try to repair almost everyhing (e.g. by welding it).
You can always go to
www.redlineweber.com if you are looking for a (good) present for your son.
I am now facing a problem similar to yours. My sunny B310 (US 210) carburator has a leak in the buterfly axles and I was asked the US$150 for the throtle chamber. So I am going to dissassemble it and it is going to be repaired almost for free.
Aditionally, i am waiting for 2 alfa romeo Weber Carburetors (they were free) for my car and then I will have to provide some new manifolds.
If you want to put a diferent carburator try to choose one from a engine with a similar displacement and with a similar power curve. With some luck you don't have to do any jeting.
Maybe the rest of the folks can give you a hand with this, since there is a lot of people across the US that share this passion for Datsuns.
Feel free to ask anything...
I hope it helps you
Best regards
Ricardo