you have an 87 4runner, so why are you putting the different rear axle in from the 89? they are the same width. if you are only doing it for the v6 3rd member then you dont have to change the housing just the 3rd member. 4cyl and v6 3rd members swap out just fine.

According to the website, it sounds like a plug and play operation.  do you know who you talked to at Marlin?  Ask for John or CJ, they should be able to help you answer all questions.