How can I clamp two 325 mm planks while gluing them together?

If Bessey K style are not in the budget, Bessey H pipe end clamps are very economical, with new lengths available for the price of an iron (or steel) pipe.

Found these on Amazon UK

What you're wanting are called "Sash clamps". Some people call them bar clamps.

You can join them simply with a good PVA glue. But personally I prefer to dowel them as well for added strength. With this sort of length you'll want to put at least 3 clamps on the bottom and 2 on top to help keep it flat and wind it up tight to make a nice wide board.

The process looks something like this:

Edge gluing boards together

Make sure you put clamps on top because the pressure can cause the wood to bow out. Clamping from both sides prevents that bowing and keeps it nice and flat. Note the edge protector! that's important if you want a nice finish.

They come in different lengths usually around 1+ meters. They are probably around US$20 each.

I had a bunch of short hand-clamps but needed to clamp something much wider.

So I used pairs of clamps with a length of binding rope to achieve a similar effect. It wasn't as tight nor straight, but worked well enough.