If you are using a powerfull .46, and it looks as though you are. You will probably need more downthrust than that built into the firewall. I have one built from a kit with the TT .46 it needed about three washers under the back of the engine to run right. With a K&B .40 it was alright without the washers. IMO the Thundertiger was way too much power. The K&B is a bit underpowered on a hot day, a modern 40 (not LA or GP) should be about right.

Suggest you fly it without the cowl and adjust the downthrust before correcting the fit. Who knows you may need to add a few washers to the front of the engine.

Another alternative is to mount the engine higher. This has a similar effect to downthrust and may make the cowl fit better, if the front of the engine isn't too high.