rajeev,

i'm not sure if I understood exactly what you want to do, BUT if you just want to "test" IF the thrust line will be affected by changing the mount orientation you can do this:

1- place the engine on the mount but with the shaft (where the prop goes) facing backwards almost touching the firewall, and secure it using a drop of CA or some zip-ties
2- stick a piece of paper on the firewall
3- mark the place in the paper-on-firewall where the shaft touches, maybe draw a circle with a pen around the shaft on the paper
4- remove the screws that hold the engine mount to the firewall and rotate it to the position you want to achieve
5- see how different the position of the shaft is in relation to the circle you draw around it on step 3

if there is no change then go ahead an mount it in that position and save you the drilling of new holes

Hope this helps

Joe