I will give ya a bump here and by no means am I a XR expert

The nice thing about the XR block is.........

its a wet sump
takes stock pan
stock timing cover
has factory bell housing bolt pattern


as far as the lifters and cam go you would need to buy $$$$ pricey race stuff anyway to take advantage of the P7 heads anyway. A stock cam would flex and could not be ground to take advantage of the heads anyway.

the used R5 lifters are dirt cheap if you want good used stuff.

its not perfect but it will let you build a big inch P7 headed motor with out breaking the bank