Colin;

Volume for a radio is quite limited in an 1800...and depth too...even the stock radios required a wedge be installed between dash and radio in order to drop the tail end of radio to clear wiper linkage!...I'll try to dig up a pic...

If you want a modern sound system in your 1800, in preference to hacking up the poor dash, you should consider a remote base unit (mounted under the dash, or seat, where there's plenty of room) with only a thin control head mounted up there...I've got a totally stock AM/FM mounted (with OE wedge), and a modern sound system located covertly...best of both worlds...

Cheers