A500 parts interchange with 904/998/999

What parts from the 904 family (904/998/999) will fit / work in a A500 ? Also is the (22.5 degree to 15 degree) overdrive planetary update. A needed thing ? or just a smart thing to do will your in there ?
For 300 HP would more frictions/steels be "needed" in the front on the trans, and the overdrive unit also ? For 300HP what mods would you think are really a must ?
What about the plastic servo in the A500,will a 904 servo fit/work ?
I've also seen to increase cooler lines from 5/16 to 3/8 or larger. But most coolers are set up for 5/16. Is that an issue ?