Well it sounds like you've decided to go for a 360. If you find a package deal on a 360 with trans, go for it- it'll have all the bits & pieces already to make them work together.
If sourcing things separately, get a mid '70s or later a904 (or one of it's variants: a998, a999) as it will have the part-throttle kickdown on it. It will bolt in in place of your slant six 904; and use the same driveshaft, shift linkage, cooler lines, and speedo hookup as your current transmission. If it was originally backing a 360, you can use the torque convertor and flexplate that comes with it. If it came from a 318 application, you can use the 318 torque convertor with a B&M flexplate #10239 (to balance it to the 360 spec.) This all only applies to LA360s, not Magnum5/9/360s- they're a different animal.
This will allow you to continue to use your (probably, but confirm this for yourself) 7.25 rear end, at least for a little while...
Don't use a trans from a magnum (overdrive) unless you're into frame surgery and structural fabrication.
Exhaust manifolds are easy- any '67-76 A body 340/360 manifold will bolt in, Magnum manifolds will fit with a little tweaking/grinding... Member
@318willrun has reported good results using 360 B/E body manifolds, too.
