the Buick 215ci shares the Land Rover v8 bellhousing. because the Land Rover 3.5L to 4.6L is a metric conversion of the 1961-63 Buick small block. so, you want to get ahold of your local Land Rover mechanic / fanboy and pick through his parts pile.
otoh, you can go to www.AluminumV8.com , they have a manual conversion bell ( Ford pattern, but GM used quite a few of those ) set up for hydraulic throwout.
Did you find a realistic option for your 5-speed? I just got an import with a ‘62 215 Buick and a lightweight (rated max. 245 ft. lbs. torque) Borg Warner T50 five speed.