Not saying mine is right since it still is sitting there as it looks but just for thought here is a pic of mine
Also i think PEESALOT hit it on the head pretty darn well...
You will see i put mine flush with the front of the frame but only because i was going with a Scout PS box rather than the regular saginaw. I would be pretty nervous not extending the frame a little if i wasn't. (NOT PROVEN JUST OPINION)
I just remembered are you planning to use a rod or something to help ensure you are square on your back tubings?? I will find my other pic to post to explain if you are not. (i am sure you are but i'm just trying to help)