78 Dually donor for my 68 GMC Dually

[My Alter Ego]

Ok, so I found a 78 Chevy Dually donor to use in updating my 68 GMC Dually flatbed. I've read (and will re-read) a number of threads regarding swapping out the front end from this 78. What I'm still unsure/unclear about is whether or not the rear end from the 78 will be a direct bolt in on my 68. I guess I'm concerned about the spring perches being in the same location between the two years. Will I need to cut the perches off the 68 and weld them onto the 78's housing? Are/is there anything else I should be aware of. I won't be picking up the 78 until this Friday so a can't measure anything for comparison purposes. Any help, knowledge, tips or suggestions would be greatly appreciated. Thanks in advance.