91 chev 350 intake issues

have an older style block with centre bolt heads. replacing the intake and curious why the rear water port is closed off in newer model 87 and up and not prior to 87. do I need to close off or leave open with a 84 block and 91 style heads?

are you refering to the hose lines that lead to the heater-core?

no referring to the intake manifold gaskets

Then what do you mean by "rear water port"?

1987? Or do you mean 1997?
The early intakes that have a rear port heater supply fitting have open gaskets. Later engines that have the heater supply in the front coolant crossover have the rear water port block-off gaskets.
The intakes also have internal water jacketing to cool the e.g.r. passage.