I have done many Weber conversions, but I'm currently away from home so I don't have photos to post. I can send you photos Sunday is you would like? If your referring to a Weber 38/38 syncornis (spelling?), that is a lot of carb for a 258, but it can work.
Most importantly you'll need to reduce the fuel psi to a constant of 4-5 psi. A single feed with a quality regulator will work, but you may want to consider a return line utilizing a Holly regulator/ return valve. Weber are dependent on a high volume/ low pressure (but constant) fuel supply.
But back to your vacuum question. Your distributor will be connected to the base of the carb, it's a 5/16" hose barb (ported vacuum). On the adaptor supplied with the carb there should be a larger 3/8" hose barb (below the venturies, full vacuum), this will go to your PCV valve and of equipped your p/b booster. The other end of your PCV valve (the inlet/suction will start at the bottom inside of the air cleaner). Most of the time with a stock manifold setup, there will be a threaded brass fitting (bulkhead) just above your ERG valve on the intake manifold that is connected for vacuum to your various emissions controls, somethings the p/b booster is or can be connect here.
Hope this gets you on the right track..... keep us posted!
