A few general comments from a beginner.

For HPDE, HSAX and track days you may like to have a cage but not required. The cage is required for wheel to wheel in general.

As far as wheels and tires, there are some dedicated track tires in 15 and 16" but very limited selection and not much of anything in an ultra high performance road tire (eg 200 TW). With the right tires, your current 18/19 wheels may be fine if they are not too heavy, especially if you want tires that work for both road and track.

On the engine, are you starting with AL heads? If so compression in the low 10's should be safe for sustained hard running and a cam in the range of 230-240 intake duration @.050. The engine deserves its own thread with more detailed info. For handling duty, keeping the oil under control is important. I just changed to a Milodon road race pan for this reason.

If your 5 speed transmission has the long OD (~71 ratio) it will functionally be a 4 speed at the track and that should be fine as long as rear ratio is dialed in. I wouldn't see need to change it.

In terms of rear axle, an 8.75 will have adequate strength and be a little lighter... but I would not make changing it a priority. Not a big gain IMO.

Not sure about your suspension and brakes. There are plenty of threads on setting up T bar/rear leaf if that's the way you go. On a medium or larger track, the brakes will get quite a workout with multiple laps. Cobra style 13" discs in the front and 11.7" discs in the rear gets it done for me (after I burned up the rear drums).

I guess there are lots of ways to get started. I went to an all day driving school sponsored by the NSSCC (club). I guess I drove well enough that they invited me back. I followed that by another full day. HPDE in the AM and 8 timed HSAX runs in the PM. Loads of fun.