A gap of 1/16-1/8 INCH would be fine.

If this has a "clamp type" mount:
1- try to assemble to motor/prop/spinner assy and slide it into place. May need to assemble in place in some a/c.
2- Loosely install the clamps so the engine can slide a bit.
3- position the engine assy for desired clearance.
4- tighten screws in clamps

If not clamp type, do steps 1 & 3. Then mark where holes need to be drilled. Remove motor and drill holes. Mount engine.

You can usually leave the muffler off until last but not always