You might check out Performance Options in Oakland (They have a webpage with a Celica on the rack) and talk to them about maybe swapping in a later model Toyota motor and a much bigger radiator.
When I had my early Chevy I learned that a bigger radiator was a must have option and it also helps to make sure ALL the air coming in the front of the van is going THROUGH the radiator. (as opposed to around, over or under it)
If you're getting blown around in the wind, wider tires will help.
I don't know anything about Performance Options, but they look like they would be a good place to start asking questions....