The c4 should be everything between around 150 and 330. C4 150pf result in a better bandwidth, if I remember well, than 330pf. The goal here with the vari cap is to put the unit on an analyser, feed it with a signal and adjust c4 until you get the better response possible. In most case, the value ended up being around 150pf. But if you just have 330pf, you can go with it and change it latter. Looking at the tubes, you can replace the tubes and keep the 220k. Replacing them result in some more headroom.