Do you still have a standard radiator for the six-cylinder engine? Or are you running a V8 radiator instead?
Just wondering, since all the shrouds I know of are designed around the V8 radiator so would not easily install on the smaller, down-flow radiator.
Did any passenger cars with the inline-six come with shrouds? If so one of those might be a better fit.
Probably something out there, but I've not seen them. Custom is an option of course, but other than that I don't know what direction to send you.
What about the fan itself? Wasn't the 170 fan a smaller diameter 4-blade model? Or does it have more blades?
How hot is it getting in traffic, and have you verified the gauge temp reading with another method (second opinion method) to be sure you're seeing what you're seeing?
Maybe a small electric pusher fan would net you the same results of slightly lower temps at slow speeds in case you can't find a shroud.
Another option, again more of a Frankenstein method, would be to source a larger mechanical fan but make sure it's got a clutch so the 170 isn't constantly pushing that larger power-sucker up front.
Good luck. Curious what you end up doing.
Paul