I just completed a 350 swap in a 49 GMC using the stock radiator. I too discovered the engine sat to low to properly cool with a mechanical fan.
I went with a 16" electric fan with a home made shroud, a 4lb cap and an overflow bottle and so far its not gone over 205 and avgs 185-190 in normal driving and I haven't had to add coolant. I'm still experimenting with thermostats and am running a 160.
Something like this
elec fan and shroud combo may interest you.
