Mareo,
a cupple of thoughts about you problem,
the following things are what I have seen that has caused similar problems;
check the water pump impeller and make sure that it is attached to the shaft, I know it sounds crazy but I have seen them come loose and look ok, and still be slipping as they rotate. make sure that the impeller is still there, I have seen aftermarket waterpumps that had stamped steel impellers that litterally rusted off the shaft and couldn't circulate the coolant propperly.
back in 75 Nissan came out with a heavy duty water pump for A/C equiped cars, it has a much better impeller design and eliminated similar problems on A/C equiped B120's the old part number was
21010-H7226, it has probably changed part numbers by now, it will have a larger hub for mounting the pulley so you will need to get a different one, you said that you have access to a salvage yard, look for one with a fan clutch, that's the heavy duty setup. if you can get one, you don't necessarily need to use the fan clutch, you can get a universal fits-all fan and put on it.
the other things that will cause a problem like you have are a carb that is too lean or spark plugs that are too hot in the heat range, or possably both. it might be a good idea to get a new set of plugs that are the correct heat range, go to your favorite autoparts store and ask for NGK BP5ES, that again is an old part number, but they should be able to cross it, either that or go to your local dealer and get some, they shouldn't cost you any more at the dealer.
also, if you are in serious need of a good carb, contact me off list, I have a line on something that might help.