The Letter Cars were niche cars when new. Low production, too. Neither of which equates to a lot of return on investment in a reasonable amount of time. Many of the ones worth restoring have already been done, but every now and then, another one shows up. In other words, no "quick money" in a short period of time, typically.
There are two long-time 300 Letter Car clubs in the USA. Perhaps they might be a good reference source too?
Problem can be that once you get past the items which made the car a Letter Car, all you have is "A Chrysler". Just like getting a Z/28 DZ 302 engine block surfaced. When the specific-to-Z/28 items are ground off of the ID pad, it becomes just another 4" bore Chevy block.
Just some thoughts,
CBODY67