heres where my 200 came from, nice support too. mine is stage 3 raptor (500hp rated)
youll need to get the cable adjustment precise on either. the 200 was used on buick,and olds engines and bolts right up to a poncho. should be a bolt in for the 68, same driveshaft as powerglide or standard. might need a diff crossmemeber, but tranny is same length as a th 400.
http://www.transmissioncenter.net/200-4R.htm
heres one of mine on a 455