This is a 91 Volvo station wagon with 113,000 miles, auto.trans with A/C.
Questions.
1. I have been told this is a 240 model and that this is a 245 model. How do I determine which is correct?
2.I want to use my old R12 gauges on this car which was converted from R12 to r134a some years ago. I have the new 90 degree fittings to attach to the low (blue)and high(red) ports.I believe these color coded fittings will attach to the ends of my current R12 gauge hoses. Will the pressure readings be accurate, given that I will be using the gauges intended for an R12 system?
3. What do I buy to have a hose that will accept (on one end)an r134a can of refrigerant and (on the other end) will attach to the middle of the R12 gauge set (or the existing R12 hose)?
3.Where exactly is the high side port. Is it under the radiator where I saw something when I changed the timing belt?

If the system is working properly and has no leaks (or major leaks), I would suck it down and recharge with three cans of R134a.

I bought a small vacuum pump from Harbor Freight for $75 which works great for this purpose. It has paid for itself many times over. Did one R12 to 134a conversion (using the Volvo conversion kit), and about 4 recharges using it. I have found the 240 AC systems to be pretty bullet-proof, with the exception of leaks from seals that dry out, etc.

"Will the pressure readings be accurate, given that I will be using the gauges intended for an R12 system?"

Yes, the pressure readings will be as accurate as before. But the conversion to temperature will be different.
http://www.csgnetwork.com/r134apresstempconv.html

"What do I buy to have a hose that will accept (on one end)an r134a can of refrigerant and (on the other end) will attach to the middle of the R12 gauge set (or the existing R12 hose)?"

Maybe http://www.ebay.com/bhp/r12-to-r134-adapter

"Where exactly is the high side port."

The condenser (in front of radiator) is on the high side, so look for a port on the line leading from it into the passenger compartment. That port is used to check high side pressures and not to add refrigerant.

Note: If you want to add refrigerant, add it to the low side. There's probably a hard to access port on the compressor for that.
--
1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

240 is the series. 244 designates 4 door cars in the series, and 245 designates 5 doors.

So your car is both a 240 and a 245 :)