I'm a 40R which is the upper end of M and the lower end of L. Anyone here with experience with a Todd's Standard that could recommend which way I should go?
Go big. I normally wear a large and ended up needing an xxl to get around my shoulders/chest. Several have had to send their's back to get a bigger size, so the general rule of thumb with Todd's is to go a bigger size up.
I agree with Corey. If I were you, I'd go with a large. I'm a 42R and have a size large Todd's standard jacket and the thing fits me great - almost exactly like a standard cut 42 regular. It's definitely smaller than other size large jackets I own, so I don't think you'd find it too big.