5 x 1-1/2" strips would give you a 5" finished width.
To get the 7-1/2" finished you would have to cut 2" strips.
When you cut your blocks from the strip sets, measure the width of the sewn strip sets.
If your 1/4" seams are a thread or two off, then they will not be the 8" ... no sweat ... just cut the block the same as what the sewn set is and go with it.
That is providing all your sets finish at the same width. Otherwise, they need to be the same width.