Built Trans

As many of you know, my transmission has finally taken a sh*t and I'm getting it rebuilt in the upcoming week. Now is the time to throw some hard parts in there but I'm not very knowledgeable about trans. So far I'm putting a shift kit it and hardened 4th gear. This built Trans will be ran on a stock car for awhile but eventually i will be slowly modding the car down the road. What else should i include in my "built" Trans while its getting a full rebuild. Keep in mind, zzp is right down the road and the mechanic I'm going to uses their parts. Thanks for the help/advice.

There aren't many hard parts to change in the trans as long as your parts are good.
4th clutch hub
Chain/gears: either v8 chains (only replacements offered) or 7/8" setup
Have all the valve body and channel plate upgrades/mods done.
Shift kit (trans go full install, not just springs and spacers, part of it goes into the VB)
Input shaft, go full boat, not 4340

So your saying go with the 300M input shaft?

I think the 4340 is good up to 600hp though.