Trying to remember the flow of coolant - from what I have just read in RAVE and also the 87-89 manual I have (which is rather vague on the coolant path) - it seems like this (and I could be wrong)
The coolant leaves the pump
Through Block
Through Heads
Through Intake Manifold
Through External Pipes to Heater
Through Heater
Return to Pump
When Stat Open the flow through block and heads is normal, but is returned to the Rad top hose and out at the bottom back to pump....
So the way I see it - if the stat is removed or stuck open - the coolant flow would favour the rad route rather than the heater route, but you should still get flow through the heater....
It is either blocked or there is an air lock in it....
My knowledge and experiance runs out there - sorry dude!!