This is very simple just follow my steps.

1. In your map map a block: arch with size: 128x128 in Grid 8 with the following options:


2. On each side from top arch, make a new block like this:


3. Now use use vertex and align each corner to the arch, like this:


4. Pay attention to texture to not fit through block, if have like this just that texture with Texture Application and check: World


5. Delete the arch block you created on first step, select the arcade you just made and press CTRL + M (Hold shift to select more blocks at once). And on X: Put 90, and this is the result:


Now you have a nice arcade in your map, up to you how you customize