This isn't strictly accurate, but it gives a bit more of an idea of relative performance of 1967 GP cars and modern superkarts.
The sim GPL isn't perfectly accurate with the '67 cars, but it's pretty close. At the modern version of Laguna Seca, really good sim drivers can do 1m26 - 1m29 laps. The superkart lap record around Laguna is 1m23.676, set by Sam Zavaglia in 2008 (was held by Eddie Lawson of MotoGP fame before that at 1m23.875).
I can't find a real '67 GP car lap time at Laguna - some have raced at the Monterey historics, I recall seeing pictures of a Ferrari 312/67 there. There was a '66-'83 F1 race at last years event, but I can't find any details of the fastest laps set during the race. With the small inaccuracies of GPL in the car physics, and the fact that the best sim drivers use the brake and throttle to settle the car in the corners, I would reckon that a real '67 GP car driven hard could do a 1m29 - 1m30 at Laguna, 5-6 seconds slower than a superkart. Either vehicle would be awesome to watch there! Sam Zavaglia's fastest lap onboard:
http://www.youtube.com/watch?v=O2o--m0wsu0
Also for a closer comparison to '67 GP cars, Bobby Rahal set a 1m25 lap in qualifying for the 2008 Historic Can-Am race at Laguna Seca, in a 1972 Lola T310 - 8.4 litres, 900hp (!). To show how fast the car is and how quick Rahal still is, at the 2010 Monterey historics the fastest lap in the IMSA GT/GTX race was a 1.33.02 by Rob Walton in a 1978 Porsche 935, and fastest lap of the weekend was a 1.24.79 by Stefan Johannson in a 1986 Porsche 962.