1973 Austin mini stereo setup?

I will try to be concise.
I am going to put a stereo in my 1973 Austin Mini. I already have the head unit, 2 amps, and speakers.
There are 4" jvc's mounted to the wood dash (no boxes behind them), and 6x9's mounted under the rear parcel shelf (I am going to put boxes under these).
The head unit is: Sony cdx F7710 (52wx4)
The amps are: Sony XM-504z (500w) 4/3 channel and an old Panasonic CY-M5002 (bridgable 2 channel 100w)
Now the questions begin:
1: The front speakers don't have enough room behind them for boxes.They are flush mounted into a wooden dash. Is there anything I can do to improve the sound (packing...)?
2:Should I just use the 4 channel amp? Should I use one amp (panasonic 100w) for the front, and the 4/3 channel (500w) for the rears (as a 3 channel) and sometime get a sub.
3: should I get a sub right away? If I use the sony 3 channel amp (250w to the sub), what size sub..
Thanks for reading this. I know it is low end, compared to most of what is discussed on here..

here are some of the gear:

1. you can try poly bagging the fronts...its an old school trick I learned. basically using a plastic baggy to act like an "enclosure" and stuff it with polyfill.

2. I'd use the 4 channel first and see if you like the sound of how it is...then later use the 2 channel for the sub.

Thanks. I am glad there is something that can be done in the front...