Don't forget to try the 1952 Le Mans winner - Merc 300 SL and 1959 winner - Aston Martin DBR1/300 (Shelby / Salvadori) - there is a big difference in performance between these two cars, but they can make for some good multi-class racing - for instance try using the Merc, but turn down the AI so that you are battling with the Aston's or do some longer races in the Aston and enjoy lapping the Mercs.
The Group 6 cars in the game were both Le Mans winners (Porsche 936 and Renault Alpine A442B) and are lovely on the historic layout. The 935 was also an overall winner - race this car and pick multi-class Group 6 and 5.
Glad you like the Lotus 40 and Porsche 908 - I also love these cars, but find it disappointing (immersion killer) that they are mixed in with the Ford Mark 4 and Ferrari 330 P4 as they were not bedfellows! The Lotus 40 was only used for a very brief period as in real life it was such a poor car - it was designed for lucrative races in the USA which in 1966 became the Can Am series - it should really be with other period cars such as the McLaren M1a, Lola T70 and Chaparral 2a - sadly we have just the Lotus 40 in the game.
The Porsche 908 is another odd decision from SMS - they gave us the version without lights, because it was used in daylight races in preference to the 917 when a light, nimble car was required e.g. Targa Florio and Nurburgring. Try it on the Nordschliefe and at Bannochbrae - probably the closest you can get in game to the feel of the Targa Florio. A version with lights would be a fraction less realistic for some races, but would be a lot more useful. It definitely shouldn't be in Prototype B with the Ford Mark 4 and Ferrari 330 P4 - SMS we definitely need that custom championship feature to eliminate inappropriate cars from offline races!