I'm replacing the cab mounts in my F350 dump truck. The radiator support and rear mounts were easy because of the amount of access they have. Simply find a rod the right diameter to barely go inside the top flange and get a big hammer. The front cab mounts are trickier because the firewall angles over them and they are in a compartment, so direct over the top access is denied. I got a length of 5/8 threaded rod and placed two ny-lock nuts back to back on one end. This gives the acorn end of the most inside one towards the lip of the bottom flange on the mount. The idea was to then use a washer and nut to make a puller with a piece of 2x4 and a 2x4 spacer against the frame mount (which looks 'triangular' when viewed from the front) and just crank on the nut until the piece pulled out. No joy. You get to a certain tightness and the whole assembly spins. But there was a length of 2x4 sticking out (part the threaded rod went through) where I could hit it with the 4lb hammer. Problem solved. The upper and lower flange's sleeves lock into each other so the mounts would stay on the frame brackets with out there being a bolt through it. There are two teeth sticking down from the upper mount that lock into two features coming out of the sleeve of the lower flange right at the flange part. Pretty clever, and quite the PITA to get apart unless you have a lot of force. Hope this helps.
Now my issue is the cab shifted as it is parked on an incline, so I need to get some friends to manhandle it back to square on the body mounts. Let that be a lesson. Replace your body mounts with the truck level.