Yes, the cowl is a too tight of a fit, but depending on your engine/muffler configuration you are probably going to have to make a cutout on the bottom of the cowl. Doing the cut-out first will definitely help alleviate some of the stress on the cowl. If this isn't enough, it is also possible to make a neat slice in the bottom of the cowl to permit expansion. Spread the cowl a little, then use some CA hinge material and CA glue to make a little bridge for the gap.

These things always take longer than expected to assemble. (One exception: my AW Yak 50cc that took 10 hours flat!) Keep in mind that you are making some significant modifications, which always complicate and slow down the process. Even little additional things such as painting/tinting the canopy can end up taking a couple of extra hours.

When you are finished, the most important thing to do is to SLOW DOWN, use the checklist in the instruction manual, and to get another knowledgeable pilot to go through it with you. Even for someone with lots of experience it is very easy to breeze over items in the checklist without really critically looking to see if they are done right.