Blocked muffler/ No muffler

[mtruckr]

There just seems to be 'something' holding back the truck... we're thinking it may be a blocked muffler passage. It might be original ('69 3/4 ton), and we get a nice blast of black from the pipe when it gets revved in 'park'. It's a 305 V6, so just one pipe. (We are aware of the limitations on that motor. )
Other aspects have been tended to... it's been on the road for a year. Starts and runs fine...

I'm thinking of simply removing the muffler (just the muffler) and running it for a few days with nothing... is that unreasonable? Is it gonna be THAT loud?

Thanks-