Any chance you can post a picture of the engine serial number? I suspect it might be 48Z111695, which would make sense from a Buick perspective. I've attached a picture of a similar engine number stamp from another 1968 Fremont car (48Z101820).
48Z111695
4 = Buick
8 = 1968 model year
Z = built at Fremont, Ca
111695 = sequential number, range for Fremont was 100001 to 120752 for V8 cars
I don't have a picture of a 1968 replacement engine, but I've attached one from a 1969 car, decodes as follows:
B9E61802
B = Buick
9 = 1969 model year
E = Engine plant
61802 = sequential number
I've also attached a couple pictures from 1970-1971 replacement engines as well.
http://www.v8buick.com/index.php?thr...-block.167034/
http://www.v8buick.com/index.php?thr...nd-etc.317301/