If you want a bolt-in, the 340/350 Buicks will. The existing transmission should be a T14 which were some of the better Jeep 3 speeds.
If the future use plans include trail riding, a 4 speed would be the best option and for those, the SM465 will be the shortest adapted length. While the Buicks bolt in, the parts selection isn't that great and the prices will be more than a Chevy.
As for the merc, it may weigh a bit less but power wise you would be better off keeping the 225. With a cam, headers and a small 4bbl, pretty easy to pull 200 hp from one.
What would I do? There would be the ubiquitous Chevy 350 in it with the SM465 and I would be keeping the D18 and add an overdrive. Best of all possible worlds. I like D18's. They can easily handle V8 power, give you the best rear driveshaft angle, have the option for a bolt-in overdrive, and keep both diffs in-line which, at least for me, makes picking lines a lot easier.
Being a 70, does it have flanged axles or 2 piece? The flanged version appeared about mid year and is the best factory rear axle ever put in a CJ.
1947 CJ 2A, 1949 CJ 3A, 1955 CJ 3B, 1968 CJ 5, 2012 JKUR