It is a dead end.
I did just that years ago and am back on the 73. the 306 does not lead to a 330/340 job as you would apply non rated like everyone else. Heavy hours on an obsolete type does not look better on your CV than the 73.
This is probably not even long haul?
From those who flew with me, most are now on the 73, one on a 75 (all of them had these ratings before) and some are with EAT now.
If you can get your employer to let you fly both at the same time, that would be different. Or at least negotiate to keep you current on the 73...
Very nice to fly, though.
good luck!