It's a VHS tape called "Hot Cars", and one of the sections of the tape is on the GT-40, and they showed vintage Stirling Moss footage of him driving a Mark-4 running the old original LeMans and hitting upwards of 240 on the Mulsanne Straight.
That same video is on the tape.
He's above 150mph at the START of the Mulsanne straight, and above 200 just a quarter of the way down the straight. By the time he hits the end of the straight, he's into the 240mph range for just a few seconds so it's safe on his tires.
EDIT: Here's another
In the rest of the tape they explain that in race trim, the Mark 4's were geared to run 240+ in top gear flat out. Powered by a big block 427cid V8 bumping well over 650hp and some very slippery aerodynamics this is easily possible.
Those crazy speeds were the main reason they put the chicanes in the Mulsanne straight, the cars were getting too fast. The same thing happened in NASCAR, those old Daytona Superbirds were pushing 220+ way back in the late 60's and early 70's so NASCAR made a ton of rules to slow the cars down.
Also, contrary to popular belief, the Veyron wasn't the first "production car" to hit 250+mph. The Callaway "SledgeHammer" twin turbo Corvette did it first, it hit
way back in 1988. I still have the car magazine that ran the article somewhere.