I would go with a 4L80. Its not about fuel milege but being able to haul something with lower rpms so its easier on your engine. That trans is strong and durable enough in the long run all new heavy duty chevy trucks run them. Thats why you dont see 350s or 400s because the 4L60s and 4L80s replaced them. As far as the electrical harness part you can get aftermarket from Summit Racing and its not that hard to hookup to your existing harness.
