G'day Alan,
yep a 400 would be pretty close. I run a 32mm concentric on my '51 A7 ('57SS top end 357 cam and straight through exhaust no air filter) with a 420 mains.
Dutch, the car hit the gutter (dirt road) the trailer jumped it and side swiped the tree. All at 60MPH. The XT didn't budge and was OK.
Cheers