There are three things that you should consider:
1. The parking spot type. If you want a plane to park at a Gate, then the you need something like this in the livery's aicraft.cfg section:
2. You also need to check that the radius of the plane (in FS9 you can check that using AFCAD/Tools/Aircraft Editor, in FSX it's 1/2 of the wingspan value in the aircraft.cfg file). The parking spot must be larger than this.
3. If you want the plane to park at a specific gate, then you also need to check the following aircraft.cfg line as well:
The line above says "if the parking spot also has the code AAL (American Airlines) then give me priority parking there".
So you need to look at your Spitfire and your 737 and see if they already have a parking code listed for that livery. If they do, make sure that the desired parking spot also has that code (they are case sensitive). You can have parking spots with many codes, BTW.
If the planes don't have a parking code, give them one for the correct airline (or military type) and give the parking spot the same code. If you can't find out what code to use, you can just make one up (up to 4 letters long).
Hope this helps,