To be honest, if clearance is the only thing you are worrying about, then you have all your priorities wrong.
971 always starts with the following document:
http://www.gates.com/~/media/files/g...nual.pdf?la=en Bookmark it and read it. It is awesome and we've found it pretty accurate over the years.
The biggest challenges in a belt drive system are belt life, strength, and proper tension. We've struggled most with proper tension over the years. We also try to design for 0 belt failures over the year, since the drivetrain is such a key part of the robot. Our year is 3 competitions, and 3 offseasons on one robot, and many many weekends of practice on the other robot.
Lets work through some of the numbers. I'm going to start with numbers from 971's drivetrain, which I know well, and then lets extrapolate.
For the last couple years, we have been running 24 tooth pulleys with 3.5" wheels. In 2012 and 2013, we ran GT2 belts, 9mm wide, and they wore out by the end of the year and needed to be replaced. In 2014, we ran GT3, 9mm wide, and when they weren't perfectly tensioned, they broke. Too little tension and the teeth ripped off. To much and we broke the tensile element. It was a fine line. To me, that meant that we were on the edge of what they are capable of. We've since moved to 15mm wide GT3 so we have some safety margin.
Lets pick GT2, 9mm wide as the 971 suggested torque without a safety factor.
GT2, 24 tooth, 9mm wide -> 75 in-lb of torque.
Stall torques for the other configurations we've used in the past:
GT3, 24 tooth, 9mm wide -> 86 in-lb of torque.
GT3, 24 tooth, 15mm wide -> 159 in-lb of torque.
For grins, lets look at some other numbers. VexPro uses the HTD tooth profile.
HTD, 24 tooth, 9mm wide -> 36.9 in-lb of torque
HTD, 24 tooth, 15mm wide -> 70 in-lb of torque
You can run GT2/GT3 belts on HTD pulleys, and you will get performance somewhere between the two (Gates won't give you numbers on it, but will tell you that it is supported and is better than pure HTD). Food for thought.
So, lets analyze the proposed configuration
HTD, 18 tooth, 15mm wide -> 49 in-lb of torque
My opinion is that if you were to put that on one of 971's robots, you'd destroy the belt pretty fast. I don't have enough info from your original post to run those numbers. Wheel-size is another variable that I've been ignoring. Bigger wheels will require stronger belts.
Some more numbers to get you thinking:
HTD, 22 tooth, 15mm wide -> 62 in-lb of torque
GT3, 18 tooth, 15mm wide -> 101 in-lb of torque
(If you've already run these numbers, then this post will help others understand the tradeoffs we go through in belt selection).