Hello David,
I have given to this problem a lot of thought as the original speaker layout of the 407-410 series is far from ideal. If you want to preserve the original layout you can fit one Infinity 6532Si ('S' for 'Slim') 3-way speaker in place of the original overhead ELAC/Goodmans front magnet speaker, and a single 6x9 3-way JBL CS-69 in the original place under the driver's side grille on the rear shelf. This will give you some good modern speakers; for the radio, I couldn't think of spending £2.000 on a old-style Becker but I found this cheap retro 'Caliber' radio/CD etc performing passably well for Euro 150:
Bristol 409 audio system improvements (June 2012)
I'll fit the new speakers in September (I have already bought them) and I'll let you know if the sonic result will be worth it. Fitting two original speakers in good condition, both front and back, with the new radio improved the overall sound considerably.
Cheers
Stefano