Ok, time for an update, spent all day Saturday over at mates house working on this, trimmed down the 600 front end to fit the 25 boot, and managed to get it in, stick the bumper on and close the boot. Great fun !, pictures pretty much say it all, lots of measuring and trimming and loads of angle grinder discs....
We made a strut brace first to ensure the remains of the 600 didn't go out of shape once the bulkhead was removed, plenty of strength in the lower subframes but nothing up top to stop the strut tops moving....
Then we cut the bulkhead out (cue several hours with two angle grinders going), resulting in this...
(spot your webmaster brandishing the 9" grinder)
Finally we shoved it into the 25, stuck the bumper on with some wire and tape and closed the boot, job done (well maybe not)