I cannot vouch for the complete accuracy of these values, but:
1966 MkII
Lift:
100 lbs. @ 150 mph, with 525 lbs. of drag
144 lbs. @ 180 mph, with 756 lbs. of drag
177 lbs. @ 200 mph, with 933 lbs. of drag
1967 MkIV
Lift:
148 lbs. @ 150 mph, with 365 lbs. of drag
213 lbs. @ 180 mph, with 554 lbs. of drag
263 lbs. @ 200 mph, with 648 lbs. of drag
318 lbs. @ 220 mph, with 828 lbs. of drag
pulled from Ian Bamsey's The Anatomy and Development of the Sports
Prototype Racing Car.
I cannot find any MkI/MkIB info, but I suspect it would be pretty close
to the MkII info. And note - that's "Lift", not "Downforce".
Ian