Powerglide parts are interchangeable between the 6 and 8cyl boxes. Expensive Daewoo 6 and 8 boxes had unboltable bellhousings and the chev box had the all in one cast bellhousing which still could be cut off and a new housing bolted on. There are also aftermarket cases such as Reid ( I think that's the name) but these are expensive. You can get different ratio 1st gear set for these boxes ,I think from memory the 6 had a 1.7x first gear and the 8cyl had a 1.82 first gear ratio.