No problem planing this,but you said you want
to make a nice flat and even top.

I made a 3' wide slab of 12' 2x4's laminated face to face. It was for a room divider in an "industrial style" gift shop . Fresh from the lumber yard kiln dry 2x4's can be fairly wet and my customer was in a hurry. It started out flat, even and straight.
6 months later it was not flat or even.tightening the threaded rods didn't come close to making it flat.

If you do this I suggest you use very dry 2x4's. Even then,it's going move around a bit and you will be lucky if it's very flat.

For cheap bench tops I use layers of MDF. I have a support system on mine so I can adjust for sagging and keep it fairly flat. The bonus is you can flip the top layer after a few years to get a nice smooth top again.