Hi, and welcome
There have some people who installed the speakers in boxes that are held in place with velcro on the back shelf. For security, they will install the speakers with plugs so that they can easily be disconnected and locked in the trunk. On other cars I have seen small speakers attached to the underside of the dash. The speakers are mounted in small plastic boxes that are attached to the underside of the dash with a u-shaped bracket.
Has your car been converted to negative ground? If yes, then mounting a stereo head unit will be easy. You can hang it under the dash on the passenger side, or if it has a remote control, you can mount it in the extra battery box or the trunk.
Here's a thread about modern radios with a vintage look
http://www.mgexperience.net/phorum/r...653#msg-396653
I like listening to the radio while I am driving around town (can't really hear on the freeway with the top down), or vacuuming the car or hanging out some place like the beach. I have a RB, so I installed a modern Clarion deck in the factory location and put the new speakers where the dealer put them, in the doors.