Welcome novotme!
This swap has been done lots of times. Try a search of these forums and you might also post this in the HiPo forum.
The 250 will need side mounts and you will need the correct
bell housing. The crossmember from a 54-55.1 truck will have the correct slanted mounting surfaces for the shear type rubber mounts that go on the bell and will fit your 50 frame. A later crossmember could probably be sectioned/modified to fit, as your 50 crossmember is flat.
Good luck.
