5.0 V8 in a '92 E 150 single exhaust

A few weeks ago I was driving this old van around the neighborhood to try to check out an engine problem (since fixed) when I suddenly heard the most ghastly noise coming from beneath the van. The muffler flange on the downstream end had rusted off and the tailpipe was dragging on the pavement. I got it home and took off the tailpipe and removed the clamp from the end of the muffler.

I went to Autozone and ordered a replacement muffler (Walker) with the intention of removing the front clamp and installing the new muffler. But when I got back under the van to do that, I discovered that the muffler was welded to the flex pipe. The weld seems to be spotty and is mostly at the upstream end on the flex pipe.

I think I could take a Sawzall and cut the front flange of the muffler near the muffler body and use a cold chisel or some other tool and remove the rest of the flange and install the new muffler using clamps.

Is this a no-no? I don't want to damage the flex pipe. And I sure don't want to take this old van to a muffler shop if I can help it.

Any help greatly appreciated.