8 3/4 axels???

not to steal the thread but i have a slant in my 68 dart (street,strip) thats at 200hp soon to be more like 300hp, also redoing the stock 904 with 4200 stall and manual valve body. so i have an 8.75 741 case 3.91 gear set up, just need axles and 742 case. so my questions are
what axles/bearing do i need for my hp and conditions
also do the posi's swap from a 741 to a 742,489??

thx guys