Hi, Ray....
This has come up a few times in the past, going from the "Classic" or "Vintage" gauges you have to either reproduction 356 gauges or the slightly larger ones from a Porsche 914.
As I'm sure you know, the placement and sizes of the gauges differ greatly. What is most difficult in your case is the two main gauges and their wide placement, so maybe it's best to start with a blank surface.
If I were going to do it, I would remove the dash eyebrow (it has 4 10-32 screws holding it on from the rear) then cut out the dash gauge area and replace it with a flat piece of fiberglass and just 'glass it in place. I think that Gary @flatfourfan did that on his car. Then you could get the gauges you want and cut the holes to match, where you want them. You would also have the option of either a Pre-A, 2-gauge dash or a 356 A, 3-gauge dash - your choice. I can supply a dimensioned photo of a Pre-A dash, if you wish.
Another alternative would be to place a flat material behind the dash, simply glued to the backside with Liquid Nails or something similar, then fill in the existing gauge holes with Bondo and sand smooth. I would prefer the first method, but whatever floats your boat.
On the material to use as a replacement gauge surface, I'll leave that to @Alan Merklin as I know he's done a few of these dash conversions.