Re: a couple of questions about my bummer
The rpms are about right, especially if you have a "loose" converter.
You can rebuild the case yourself for a couple hundred. You can buy a used one for a couple hundred.
You usually want to run as much backspacing as you can so you can limit bumpsteer and balljoint wear. Call up your local tire store and see if they have wheels with the offset you want with 14mm holes.