Stan, what model Rambler do you have? Pre 93 Ford Ranger axles fit the American, Explorer axles fit the big cars through 1966. 90-92 Rangers with the 4.0L have the 8.8" axle, others have 7.5". Sounds like you need an 8.8" since you have over 400 hp and wider wheels and tires. The original axle for the 196 six cylinder is an AMC 15 7-9/16" ring gear, slightly larger than the Ford 7.5", but roughly the same strength. 1964+ big Ramblers (Classic, Rebel, Matador and Ambassador) used the larger AMC 20 8-7/8" ring gear axle with the six, which is strong enough. All others (Javelin, American, Hornet, Gremlin, etc.) used the AMC 15 with all six cylinders and the AMC 20 only with V-8s.
There was a 1966 only economy fleet Classic that used a 199 with AMC 15, but I've never seen or heard of one existing. Probably did, but fleet use generally meant taxis or some big company's "run-around" car -- stripper that was run into the ground then ditched.