The AMC 360 would be a perfect choice for the Gremlin. Factory components are available to make it a bolt in job. AMC 360s came with Chrysler 727 trannys, no problem there. Sixes came with a 904, and 304 V-8s came with a 998 (as did the six in a 4x4 Eagle). Although they are Chrysler trannys, they have an AMC specific case. All internals after the bell housing are the same as any Chrysler though, so parts are easy once you have a case. Built 904s and 998s run behind high HP engines. You can get an AMC six tranny (1972 or later, before 72 the six used a smaller bell trans, 72 and later use the same bell as the V-8) and have it built for the V-8.
You have fewer parts choices for the AMC, but that actually makes building easier. The parts available are the popular parts -- they are popular because they are known to work well. There have been a good many new parts introduced lately too -- Edelbrock just came out with Performer aluminum heads. They are a little better (once cleaned up) than stock heads, but stock AMC heads are as good as Chevy LT-1 parts. Chrysler used the AMC design as the basis for their Magnum V-8 heads!