The engine in my 73 is starting to get tired it's got a 240 with a C4 transmission. The engine has over 200,000 and the tranny has over 100,000. I have a 300 to swap in but will the extra torque blow the tranny up in a few months?

 

it might, just since your tranny is old, but keep in mind that C4's were used behind a number of healthier engines than 300's, and you can build a c4 strong enough to run a sub-10 second quarter mile, so if you're concerned, or if you blow it up, just rebuild it with stronger guts.