flux gearing suggestions and high speed idler

[jediguru]

Doing a nitro to flux conversion and needing some advice on gearing. Here is my setup. MMpro, hobbyking 9xl(1900kv), hobbyking turnigy 2x2s 5200 30c. batts. I'm looking to just bash, keep the drivetrain intact, keep it cool, and have decent run times. If could get 35-40mph I'd be happy. Any suggestions?

Also anybody have any experience running the high speed idler in the tranny? I have to pick up the idler to convert the tranny and was curious when I saw the high speed idler(39) as opposed to the stocker(44).

Going to be rebuilding the diffs too. Currently running stock X diffs with 30K oil. Think it would be a good idea to bump up to 50K?