1995 Chevy Corsica black smoke & losing power

Black smoke is usually a sign of a fuel delivery problem. Chances are that you need new oxygen sensors or something that has something to do with fuel delivery.
With 230000 miles I would be warey of a mechanic who puts intake gaskets on an engine with this amount of miles. I don't know what you paid for this job but I know it is not cheap and/or an easy job.
I try to help people fix cars however, when an engine gets 230000 miles on it - and - It is blowing black smoke - and - loosing power - and - probably burning a little oil - it is sometimes better and cheaper to put a rebuilt engine in the car. I am not saying this is the case with your engine, but you should be aware of this.

Number one -- Do not change antthing until you test the item and determine that the item is bad or has failed. A mechanic who cannot perform tests to determine this is not a mechanic.
Oxygen sensors usually run between 60 to 80 bucks a piece. Depending on your exhaust system there are probably two O2 sensors -- possibly three -- located just before and after the catalytic converter - or - converters.
I have no opinion on changing a sensor until the sensor in question is tested and I have the results of the test. Is your check engine light on?
$625.00 is not a crazy amount for changing the intake manifold gaskets. I question the wisdom behind changing them in the first place. A failing intake gasket is not the normal course of action for solving a problem like you described. The first thing you should do is have the compression tested on all the cylinders of your engine. This will determine the condition of the cylinder walls and piston rings inside of your engine. There should be no more than a 10 percent difference in the compression numbers. If there is then your engine must be rebuilt. This test is easy and cheap -- have this done or do it yourself. Keep me posted---