Gasket maker (RTV) works and makes a great seal but I would get the paper gasket for an 8 3/4 "drop-out". If you ever go to swap the center section, the RTV really stucks the unit in there... making it hard to remove.
Clean the surfaces well and glue the gasket down to the housing side. I use a light coat of grease on the carrier side of the gasket and that allows the unit to come back out, with-out hurting the gasket. The allows for many gear swaps... without a nasty RTV, gasket scraping clean-up job.
If the center section comes out, check the thru holes on the case and deburr them. I actually run a drill bit thru all those holes to be sure the carrier slide onto the housing studs with ease.
With an 8 3/4... it's all about being able to do gear swaps!
DD