I like to add what Streets said. check your timing, running lean will you run hot. might have to rejet the carb.
If your running hot on the highway it dosn't matter what fan you have. Beacuse the air is forced through the radiator. But like street says check the radiator, might be to small. What temp was it running before you added the blower?
Just tow last notes, might check the radiator cap and see what pound pressure it is. If you run under a certain pressure then you system is pressureized.
Also you might have a air pocket in the system.