Would It be unsafe and would it be overloading my engine and transmission by towing a car trailer (not a dolly) loaded with a 98 Mustang with my 97 blazer 4.3 V6 about 300 miles?

 

Depending on the configuration of your Blazer the towing limit will be ~5,000-5,500 pounds.

 

For just 300 miles, you should be okay. Just watch your speed and keep it in third. I'd stay around 60mph give or take. This is providing everything with your truck mechanically is in working order.

 

Better have a trans cooler on it or a tranny will be on the short list of repairs. Everyone told me this & I burned the crap out of the fluid but luckily saved the tranny. I was towing 5000#.

 

i would take the load out for a test run and watch my rpm's if they go any higher tht 2500 or so then i would say that you are a little over loaded. i dont think the mustang and trailer will be over 5000 pounds and if it is not by much. i would say that the engine can handle it but the gear ratio and the tranny will be the things to watch.