If you're going to make the bumpers one piece, this will be interesting reading:
http://autopendium.com/vehicles/3-19...e-front-bumper
For the sound system, there is just enough space to mount a modern headunit under the dash in front of the gearstick (so the driver can reach it, unlike the factory mounting position) and old Jags are a good source of classic looking speaker covers (simple black mesh with chrome rim) should you end up locating the speakers where they would be visible.
Btw, what size are the big wheels & tyres & how did it drive?