Bolts, studs, wingnuts whatever floats your boat. Load spreaders are a must. Good thick ones, not the cheap dress up chrome bling style. Just use a good gasket and don't over torque. If your covers are stamped steel, spend some time making sure there are no dimples on the gasket surface under the bolt holes.
Well, load spreaders are a must on stamped steel or thin walled aluminum covers....
I have a set of nice OLD M/T valve covers... they don't even have space/provisions to use load spreaders on them (essentially being built in).
I plan to use tall wing nut valve cover bolts.... My rig only came with 6 out of the 8 and I liked the look... so I purposely sought after a set that were in rough shape so I can have a matching set.
With my rig... it has all of the signs that it was built "to be cool" in the 70's or early 80's... I want to sort of keep it that way.... make it a great runner, reliable, and safe... but sort of rat-rod'ish when you open the hood. We'll see how I manage to pull it off.