This is very simple; have both cooling pressure test done! At idle and at
2500rpm

http://www3.sympatico.ca/huotx/pressure_test.htm

Believe me, you have an overpressure problem and the Expansion tank cap is
not able to compensate. Very high pressure like you have is cause by a leak
at the inner gasket, or a cylinder crack head, period. The first thing to
let go is the "CHEAP" flexible outer head gasket. (This gasket is a joke,
it remind me of the V2 Honda pushrod CX-500 & 650 series motorcycle
gasket).

Let's see!!!! Other possibility!!!!!!!!
It's not a clogged front rad or any other rad., there is a bypass system on
the 2.1 that allow coolant to flow when thermo is close or any other
problem with any rad. It's the cooling tube that you don't see, it's
between the lower parts of the thermostat housing (below the Thermostat
position, on the right side!!!!) and the upper left part of the cooling
distributor (passenger side behind the firewall). Coolant is always flowing
in the Westy (at least for the 2.1). Lost of pressure cause overheating.
Same for the oil cooler, it's always flowing.

The only normal thing (apart of a failing inner head gasket or a cylinder
crack) that produces pressure in the cooling system is the heat & the pump.
The pump will not produce more psi than needed, the more it's old the less
it produces. If you're temperature is normal (should not), you certainly
get pressure from the combustion chamber

Heat is produce by air pocket, bad thermostat, bad pump, overcharge.

Good luck, Ben