You can't use the K-member or sway bar but everything else will fit as an assembly. Your existing K-member is the same for a 6 or 8 so no reason to change it. You'll just have to get some mount brackets for the 340/360 for that K-member. Make them yourself or buy them from Schumacher. 8 3/4 rear for sure but I'd use a 904 for the transmission. Have it built with all the goodies and it will hold up fine. It's lighter and eats less horsepower than a 727 and it fits the floor much better.
Have fun!
Mark