from what i understand the 200 you only have to mod the bolt in part of the cross member. the 518 you have to cut the hell out of the upper ( welded to the floor) part of the cross member. the 200 would be a lot easier swap then the 518 in my opinion.

the 200 can be built to hold quite a bit of power with simple parts and nothing really exotic.
kickdown cable or what ever they call it is available just about anywhere, carb side bracket you can get easy also. they would both need plates to fit a big block. a 518 is huge and weighs a ton. both need a custom drive shaft.