7.25 to 8.25 swap? /6 to 318 swap?

Yes there is a bolt in A Body 8.25 rear. You can find them in the 73+ BBP cars. A neat trick is with the 8.25 rear is you can quite easily install rear disc brakes. Just find a 96 and older Jeep Grand Cherokee and take the rear disc brake setup of and it will be a direct bolt in to the A Body 8.25 rear. You can also take the ring and pinion & limited slip unit and get better rear gear ratio into the A Body 8.25 rear. All for dirt cheap money from junk yard parts. That said though, when you find the A Body 8.25 rear make sure it has the 2.94(or higher number wise) because the 2.71(or lower) uses a different housing.

I personally have no problem with swapping motors that said a junkyard LA roller Block (late 80s/early 90s vans and trucks) or mag block is what I'd use. Which can be had at a junkyard on the cheap. If you do decide to swap/or just install a V8 you'll either need conversion mounts or swap the K frame (not that hard to do and a great to rebuild/replace worn out bushing/parts), if swap k frames which is what I'd do, try the get your hands on the 73+ spool mount version because it's a much better design.

As far as the transmission goes I believe you can use a truck 727 tranny in an A Body but don't quote me on that. Personally I'd use a 904 over a 727 because the 904 is smaller lighter and uses less power to operate than a 727 and a stock 318 with black holes for pistons could use all the power savings it can get, but that's just me.

Not sure if this helped or hindered you but just giving you ideas etc... but I'd go V8 over a slant 6 anyday of the week! Oh and before I forget, ditching the 7.25 for a 8.25 (or bigger) is ALWAYS a good idea! The 7.25 rear is the biggest POS ever and even a bare bone stock slant 6 can break a 7.25 rear, just sayin.