If I understand, there was never a return air HVAC line from the room? Just a supply?

Our code states an exhaust fan 300cfm or greater requires a make up air system. Not sure about a supply though being that high. Either way you are under 300.

You could have used the blower as an exhaust out of the room at the rear and had a motorized damper interlocked so it opens when the fan comes on to let outside air in. This is how parkades are exhausted. Then run the fan off a close on rise thermostat. (A/C t-stat) This way your HT will heat the rest of the basement and you dont have to open any drywall outside the room. You can still do this, just put another blower in the rear of the room and have them run off the same thermostat in parallel. Is there space above your equipment rack to hide it all?

Man, it sucks you have to revisit this in a finished space. But it must be terrible enough to make you do it. Hope it all works out!