Had the same problem 2 months ago with my 1984 245. My car had the crossmember shims in place and did not have floor damage. First attempt was to bend header pipe in place using a big lever. Things moved, but pipe did not bend.

Finally took the header pipe off, filled it with sand using a lot of tapping to get it to pack down. Ends of pipe were plugged with wads of newspaper tamped tightly in place.

Made a measurment tool to check bend progress by marking a spot on the floor next to a wall for the flange at the engine end and a mark up on the wall for the cat end.

Used my Harbor Freight 12 ton press with soft wood blocks on both sides of the header and made a couple of presses resulting in a bend increase of 1/2" at the cat end of the header. The sand and the wood blocks prevented the pipe from collapsing.

Removed newspaper and sand and reinstalled the header. Now the clearance matches that on my other two 240s (roughly 1/2" at the closest points).

Good luck,

Tatra Mike

1984 245 "Buster" (off with the kid)
1985 245 "Cosmo" (at anchor in the drive)
1985 244 "Alfsen" (plugging along)