SM465 with a 10-spline can ONLY be mated directly to your 4-speed case with a AA adapter. the input on the case is 16 spline. Other option is to use the Classic Cruisers plate and switch to a 73-older t-case.
IMO, either the AA doubler or the Marlin doubler are the hot ticket. Both have advantages, and I know people with both that are very pleased. Road speeds are untouched, and you get lots of low range options. Only issue can be rear shaft length, but swap rear springs end for end and the 3.5" length change should take care of any short-shaft issues.
The banana rating for all the options is relatively the same, each has differences and issues to work thru.