A833 versions - how to pick?

I’m daydreaming about a conversion kit...Brewer’s performance offers a complete kit and it looks like Passion may have most of the parts too.

I’m really at square one here, hoping for some wisdom regarding the A833.

I’m going for backroads driving fun...which probably would favor closer ratios.

Overdrive seems like a waste with “only” four gears...but it sure would be nice to save a tiny bit of gas...jury is out on that one.

I don’t anticipate running a blown big block at any point but a hot small block is certainly a possibility down the road...so I’d want it to be reliable with ~500hp.

I gather that there are aluminum case versions...are these strong enough for performance applications?

What do I need to know about 18 spline vs 23 spline? Are there other spline count versions?

Thanks for any advice.