Most would go for the Penske Racing MKIIIB that won the Daytona 24 hours in 1969 raced by Donohue/Parsons.
Otherwise, you could go for the John Surtees Can-Am T70 Spider, which won three races and the first Can-Am championship in 1966. The car is much redder than the following photo shows.
Cheers - and good luck with the T-shirt.