need some input on a budget 318 build

Timo, welcome aboard!

The exact depth of the piston should be measured. This would be in thousands of an inch. (.050 or .080) You'll need the proper gauge for this. It kind of stinks because the American way of measuring these things is different than how you do it. I do not have a conversion table at the ready.

Compresion is a key item to have and part of the combonation that makes things tick.

I would use 1-5/8 tube for headers.
Can you or will you change the gear ratio in the rear? And the stall converter, will it remain stock?