The single gauge strap mount under the dash is best. Try Mike's or Mac's or Lebaron Bonney or one of the other vendors. Or use the Parts Wanted side of the Swap Meet on this forum. You could always raise your hand at your next Club meeting and ask your fellow Model A'ers. We be blessed! Lottsa sources for parts.
I like the two-holer that mounts to the steering column, also sold by Snyders. Then, you get to decide what other gauge you want as well. I've got a tach. Oil gauge is popular too, but a voltmeter probably makes more sense.
There are 2 sold. I had problems with a 31 indent firewall where the rebuilt shift tower hit the gage. Instead of buying the right one I rebent the shift lever. A rough solution to a simple problem. The 28-8 instrument panel has higher position than the 30-31.
I use the double bracket that fastend on the steering column and matches the bottom of the gas tank, fits well and I use it for temperature and oil pressure ( B motor) when hot has 15# oil pressure Gord. B by the Bay
Attached photo (I hope) of a temp guage I installed in my Victoria. It tucks up in the corner of the dash rail. The little push button switch turns on a light inside the guage if you want to take a peek while drving at night.
I made a 2in.long by 1and 1/2 in.wide sheet metal strip and bent it to a 90deg. the long way and J.B.welded it to the back edge of the dash rail.The bezel on the guage bolted to it nicely.I put it as far left as i could.Itlooks good and very functional