Transmission replacment questions?

So my truck just rolled over 700k today. Been thinking about it for a while but decided that its likely time to start looking in ernest for a rebuild or replacment on the transmission especially as i know its pretty worn just based off everything else wrong ive had to fix from the previous owner. And currently im leaning towards replacment just to swap to a much higher torque rated unit so i can ploink a 510 rated MX13 until i can afford swapping to a 6NZ or nee truck entirely (Im considering a 1850 or 2050 model) and seeing as this is my first rig i have only a general idea of what im looking for.

My questions are aside from obviously the Ujoints, bushings mounts flywheel and driveshaft what else should i have checked and/or resurfaced/replaced? Additionally seeing as im going from an eaton RTLO-16913A to a RTLO- 18 or 20 913A will my PTO transfer over or am i likely to need a new one?

 

Its more all the metal shavings ive found the last 2 times i did a PM on it that has me worried. As well as the fact the first time i did a PM after i bought it a black sludge poured out instead of oil. Methinks they never once changed it in 500k miles lol....so i dont trust that the transmission is in good shape.

 

Oh i know on fine shavings but with me having done them at 500k 600k and 700k and getting a rather large amount for each....well better safe then sorry especially as many were BB to corn flake sized. Im also fairly sure i have a few teeth worn to nubs as 3rd gear likes to pop out on its own occasionally which is another cause for concern.

Besides you know the history on this rig lol...it was rather badly abused.