first question: did the bolt pattern on powerstroke 7.3's EVER change?
second: What are advantages and disadvantages of a dually rear axle and when would it be needed or help?

 

are you talking about the wheel lug pattern? the ford 8 lug went u changed from 1966 i know for sure all the way til 1997. in 1999 when they came out with the super duty. they cahnged it just a bit. i think it is 1/2" bigger. on the early 450 and 550's it was a 8 lug but it was a noticable difference in spacing. then thay came out with the 10 lug.
a dually axle is almost pointless IMO unless you have dually wheels to go with it along with dually fenders or a 8' wide flat bed.

 

they went from standard ta metric in 99

 

sorry i didnt read the first one over before i posted. i'm talking about the bolt pattern for the trans to bolt up to it

 

bubububump