When used on 49 to 53 blocks you have to use the truck bellhousing adapter ring. (see pictures, it's the grey bellhousing bolted to the block).
These are replacement adapters that were made when I restored my engine stand. Hard to find originals that are generally missing. See more stands for sale then adapters.
I also have a stand with adapters for sale in another listing. This item is in the category "eBay Motors\Parts & Accessories\Car & Truck Parts & Accessories\Engines & Engine Parts\Other Engine Parts". The seller is "puppylove71" and is located in this country: US. This item can be shipped to United States.