Yeah, there is some thrust angle, sometimes it's built into the firewall, sometimes not. If you build it square, you can always add a washer on the top and bottom mount bolts (where the engine mount bolts to the firewall) to provide some space on the left side (which will give you some right thrust) Often, a single washer on each side will be adequate.

I'm sure someone will have more to add to this..

CGr.