Here's an alternative to the stock van instrument cluster, or even the Corvair one with all the gauges:
https://www.dolphingauges.com/product-page/1961-chevy-truck-panel-6-gauge

Jacke's 64 GMC has this. It fits quite well. There may have been a very slight widening of the opening in the dash but I don't think that was necessary. Just drill three small holes for the screws along the top. And you could use the brackets for the two outermost gauges to hold the bottom of it a bit more securely, but not necessary.
https://www.dolphingauges.com/product-page/1961-chevy-truck-panel-6-gauge

Jacke's 64 GMC has this. It fits quite well. There may have been a very slight widening of the opening in the dash but I don't think that was necessary. Just drill three small holes for the screws along the top. And you could use the brackets for the two outermost gauges to hold the bottom of it a bit more securely, but not necessary.