I read an article in scc about the same car and problem. I would check these things:

1. check fan and fan clutch ( i could of swore that's how stock sr's ran)
2. check rad. pressure cap.
3. do you still have the fan shroud and drip pan on the car assuming you still have the stock fan?
4. check the temp sending unit.
5. if you have access to pyrometer check the upper and lower radiator hoses to see if they close in temp or
6. check radiator (with engine off) for cold spots in the core. this is to see if the radiator is plugged.
6. if you have electric fans check to see if they come on at the right temp/ time, relays etc. these are some of the things off the top of my head. if you have the front end that I think you have there is not much to be had as far as air flow. if you want to read more about the Scc article go to www.SportCompactCarweb.com and look for project silvia. In one of the installments he talks about his delema of overheating and his fix.