I used to be in a similar situation about 2 years ago-- even 180cm/70kg -- no idea what that is in "stones" even though I live in London -- is quite a problem... However, what's the point of going to Japan and then buying Western cloths.
If you are looking for stylish items, like the "UK" Gap, I suppose (the "US" GAP is mid-range as there is Banana Republic above "US" GAP), A&F (if you are in the United States, go for J Crew, quality is way better there) ... and the like can be found in the Takashimaya Department Store which is in Shinjuku ... take "Shin Minami Guchi" (= New South Exit), if this is your first time to Shinjuku and should you get lost, the question for directions would be "Sumi masen, shin minami guchi wa doko desu ka?" or "Takashimaya departoo wa ..."

Good luck!