8 3/4 cut down

OK got a freebie today. An 8 3/4 from a 69 pickup with a 489 sure grip and 3:55 gears. I need to get the housing cut down to go with a set of axels and backing plates . I want this to go in my 68 dart that is still a stock 71/4 set up.

So what size do I get this thing down to and where do the spring perches go?also do I need a differant set of shock mount plates or will the 7 1/4 ones work?

the housing now measures 57" flange to flange.

any help would be great. thanks guys.