Yep, can confirm that 149 will bore out to 161 cubes.I have 40 thou oversize 161 pistons in my standard 149 which now gives me 167 cubic inches.I am running a standard grey crashbox behind it in an FB sedan alaong with a 3.08 red diff centre and very happy with it.
No problems with rego as the 149 was passed as owner approved modification ( no engineer or other mods required ).
I drive like an old fart and don't abuse my car and the grey box handles the increased power.I even tow some ( at times ) heavy loads in a trailer with no dramas.
The gears in an EH box aren't any stronger than a grey box really and you'd be going to the trouble of fitting one with no real great strength advantage gained.
The all synchro 3 speed is stronger but requires floor hump mods in addition to other mods to fit ( similar to aussie 4 speed fitment ).
The grey crashbox will take the power output of the red motors in standard form if you look after it.It won't handle a higher capacity modified red and certainly will break if you abuse it or subject the box to abnormal loading.