EDIT:Setting Line3x and Line4x modes to ‘320 x 240 optim’ is optional on Panasonic VT60. ‘4:3 generic’ will also give an image. Optim modes give pixels sharper edges though, so I prefer that over generic.
Also, on the VT60 when using optim modes, I need to tweak the various h. and v. settings quite a lot in the pursuit of correct AR. Firstly the H.samplerate needs to be incremented until the picture is sharp across the whole width of the output. Next, H.backporch needs to be adjusted for some consoles, in order to centre the image.
The final adjustment is to reduce V.active from 256 to 240 to restore scale on the vertical.
Also, thanks again for the tip on shmups and also for starting this thread.