You're probably right about having enough space to cut.

I was planning to do a rough cut just forward of the subframe (to free up the work area), then cut it again properly/accurately further forward once the new subframe is in place and I can measure up for the new centre section & tailpipe.

This thing is absolutely useless - it doesn't fit the pipe. The closest two notches on the cutter chain are no good - one is too big and the next is too small (the latter goes on the pipe but the handles are so far apart that they're impossible to squeeze together). Am I missing a trick with this tool?

I've had to make a rough cut with an angle grinder just to get the back box out of the way.