My new toy. 1966ish Valiant

Yes, a Magnum 5.2 will drop right in place of an earlier LA engine such as your present 318. The hard part will be setting up the fuel delivery and electrics. You might start out with an external electric fuel pump, but sooner or later you'll probably get fed up and wind up with an in-tank pump setup such as one of these. You'll also need to (substantially) upgrade the charging circuit to handle the (much) higher load and supply.

The OD trans can be made to fit an A-body -- it's been done -- but you'll have some floorpan cutting and mods and some crossmember fabrication to do.