Just out of curiosity, where did you get the plans from? They seem to be well drawn.
Here you go!
http://www.knifehelp.net/media/docs/GrinderPlans.pdf
I knew it had to be cut a certain way to line up with everything else. Glad I checked.
Daniel you can see on those plans, page 4 of 7 a shot of that piece! Having said all that, the 1/4" does not sound like much. So went out and measured mine a minute ago and I increased the thickness on the left side to 1/2" then I took out the 3/4" leaving 3/4" on the right side!!
Making the drive pulley, the tracking pulley and your (working wheel, or platen or small wheel) the line up of all three is accomplished by coming square off of the base and then each pulley is set to whatever specific measurement off of that square line you decide to set them at.
You will have a little to play with working off of your working wheel and the tracking pulley, the drive pulley, so don't sweat if you want to go to 1/2", it just means those other two pulleys will have to bumped out another 1/4" to accommodate for the added thickness!!
Biggest think about building a grinder that works is all about square and true!! Use sharp bits so your holes don't begin to wander and make sure when you set a piece it is square to the base!! Oh and be prepared because you are going to break some taps. Unless you have better luck than I do. By the end of the build I got real good at getting a broken tap out of the hole!!!
On that yoke I drilled and went to a bolt and a locking nut, before I cut out the 3/4" in the center. To me it was just easier to keep it all true by doing that way! Plus it saved me having to tap it!! You can use your discretion to vary slightly from the plans, you just have to understand where you can and can't do that!