(Think that was me that mentioned the racer - there was another one a year or two earlier there is a silver car... and a year or two later, a '65 Wildcat (!) which may have been the only C-Body GM car in sanctioned competition other than a demo derby.
There were plenty of X-Frame Chevrolets in the day that raced USAC and ARCA, builders probably found workarounds, thinking they were not much better or worse from a geometry standpoint than anything else. Buick 12" aluminum drums were the gold standard in home built race iron for years. Boneyards within 100 miles of any track of consequence were picked clean of them until discs were common and to this day, NOS is $$$$$$$, Most problematic chassis issue is the center bearing - they split the drive shaft in the X frame cars to get them lower and the bearing is famous for rumbling/groaning in street use, can only imagine a race setup. A manual trans conversion would not be difficult provided that the builder could score a same year standard Buick sedan, fairly sure Riv used the cowl black metal from the regular Invicta, they appear to share a common dash
FWIW Buick was quietly supporting west coast racers like Max Balchowsky (Ol Yeller) TV Tommy, Pop Kennedy, and Tony Nancy maybe a few parts leaked over to the independents. Dunno, pure speculation, but the fraternity of Buick racers was small, indeed.
Fun pic and tempting me to build!