The carbs should bolt up . . . It's probably better just to use the 150 carbs . . . clean them real good and get rebuild kits.
There are a lot of parts to make a 150 into a 175 . . . besides just the carbs. I'm not totally familiar with the mods for the Yamaha Blocks and the differences b/w the model years. You could always compare the part numbers.
Usually it's the carbs, reeds, heads, exhaust that tune the motor. The block is usually the same . . . Some Merc's have a horizontal front half and others had a verticle front half. Motors with a horizontal front half usually produce more HP. I think the same is true with the Yamaha 2.6 excel/220special and the regular block. Just ask around screamandfly.com or byuboyz.com . . . those sites are full of good info for a rainy day when your not going fishing.
Just compare the parts
http://www.yamaha-motor.com/outboard/parts/home.aspx
Ray Neudecker on boat setup had some good information.
http://www.boatsetup.com/noncgi/ulti...=000754#000000