Having a rethink now.
As I don’t have the battery in the front, there is ample space on the other side of the rad to mount the tank, however the breather comes out of the left hand side, so would be messy (and possibly restrictive) to do a 180 with the tube.
It has made me think, though, the washer bottle pipe goes all the way round the back of the block to the driver side, through the firewall there. The reservoir is right where the expansion tank would be ideal.
Now thinking I may just get an alternative washer reservoir to fit on the battery side instead and put the expansion tank in its place. Not arsed about not original, as it isn’t all standard anyway, so if I can’t turn up a Chrysler bottle, looking for suggestions as to where to source something suitable that isn’t too modern.

24 hours in a day, 24 beers in a case. Coincidence? I think not…
70 Challenger
MMA/489
NSS/435