[Cobrabill]

No such thing as "too much motor in too small a space".

Stupid stuff first:
T-stat.I've run into bad T-stat after bad t-stat.Six in all.Either failed out-right or in-accurate opening temps.
I'd install a restrictor instead of a T-stat JUST for the trouble shooting process.
Collapsing lower hose-as mentioned.
Correct pulley size-as mentioned
Correct spacing between back of the impeller and the back of the water pump.Too much space and cavitation results.
If it's a parts store pump,double check the impeller to make sure it spins the "right" way.
Radiator must be aluminum of sufficient size.It also must be sealed to the nose so that every "drop" of air entering the nose is forced thru the radiator.

All things being equal,a shroud + fans with enough CFM should in no-way make things worse.
My SPF with a Side-oiler NEVER overheats in Tucson.The radiator is Aluminum one from a V-12 BMW and has twin fans that are shrouded.Standard SPF stuff.

The above is written on the premise that jetting/timing and all other motor functions are with-in parameters.