[mikethedruid]

Yes, not only are the chassis different, the speakers are different too, electrically as well as in size. The 650 uses two 6F6G tubes working push pull in its output stage, and so uses a K35-H26 output transformer (K35 and H26 being the speaker type) . The 620 uses a single 6F6G tube working class A for its output stage, and an S7- HS output transformer (S7 and HS being the speaker type) . If the cabinet and speaker in your radio are good, ring out the speaker with an ohm meter and see if the field coil and transformer and voice coil are OK, and your chassis is not completely shot, if things UNDER the chassis look pretty good, the coils are not chewed, corroded, or deteriorated, and the band switch badly corroded, you might just be better off restoring the original chassis. A little rust on the outside is not going to prevent it from being restorable and working fine afterward. Also, the 650 is a MUCH better receiver than the 620. Consider all this, and decide how you want to proceed. If you want advice on whether the 650 chassis is restorable, remove it from the cabinet and take a load of pictures, post them to a site like tumbler, then link them into a reply to this thread so that people here can see what you are dealing with and give you a more educated estimation of what you are up against.