I helped my late brother-in-law replace the 904 in his 78? Magnum. It is a pain, but it will be easier if you nosedive it out and back in, rather than fooling with the cross member. Just make a piece of metal to bolt to the bellhousing, that holds the converter tight into the pump. It will be easier if you have a helper. My brother-in-law was kind of a wormy little guy, but the 2 of us did it easily. Getting back on the jack to put it back in after you fix it is the hard part. Other than the jack drama, it's same ol same ol transmission swap.
