Barry. As a general rule, main support girdles need to be the same material as the block so that expansion rates are the same. I’ve some horror stories here where aluminum girdles are used on iron blocks.
For the Y, the main support girdle is made from 3/8” thick steel. Any thicker than this and they get pretty heavy. The studs I either make or get through Granger or MSC simply due to ARP pricing being out of sight and still having to modify the ARP studs for the final fit. By lining up all the main caps in the mill vise together, all the caps are machined to the same height at the locating washer spots or bolt holes. All this after the mains have been align honed. Only the area around the main cap bolt holes is machined. There are equal thickness washers between the girdle and the main caps and this thickness is taken into account when machining the main caps. The main caps are machined so the girdle sits ~0.006”-0.010” above the pan rail when the girdle is torqued to the main caps. The girdle is then pulled down to the pan rail when the outer 5/16” girdle nuts are pulled down. A thin film of RTV goes between the girdle and the pan rail. The oil pan gasket is still used between the girdle and the pan.
If you’ll email me directly, I’ll send you the girdle instruction sheet. Contact info is at www.eatonbalancing.com .