I am just begining a bulid and could anyone tell me what the 7"/7.5" dimension applies to on sierra diffs. Also the diff I have does not have a thru bolt on the top mounting as stated in the haynes manual it has tapped holes instead, is this normal?

you have a 7.5" diff then which refers to the crownwheel diameter

The 7"/7.5" that most kit makers refer to is the width of the front top diff mounting width, the section the long threaded bolt goes thru. Some design their chassis to take either using spacers for the smaller version, but most spec the more available 7" which can be found in a number of ratios and with/without LSD.

I think you will find that is coincidental a Ford motorsport 9" rear diff also measures 7.5" across the top mounting lug but the crownwheel is 9" diameter

Yup, tis conincidental, but a quick idnetifier for the diffs most commonly found in a scrappy, although IMHO it is pointless using anything bigger than a 7", unless you have over 300bhp, have loads of grip, and intend to use it on a regular basis....
The diffs can either have a through bolt or tapped threads, although tapped threads are fairly rare on 7" casings.

HTH Stu.