For pulling a trailer I'd think a stock 3.9 as in most models late 49's, is as low geared as I'd want to go, even with the Mopar R-10 OD trans.
My 50 4 dr. will easily cruise at 70 with the 3.9. Not pulling a trailer. If going over a lot of big hills a 4.1 gear ratio would even work better pulling a trailer, but only with the OD.
Changing a stock type 218 to a 230 requires the 230 crankshaft and rods to be installed. If the rest of the parts are in Good condition they can be reused to save some $$$,
My 50 cents (inflation ya know!)
DJ