The need for an Engineering certificate is only necessary for modifications that, if not fabricated by a quality machinist, could hinder the structural integrity of the vehicle.
Engineering certificates are not need for a simple swapping of a gearbox as it uses the same chassis mounting points the auto box uses.
As for insurance,the only reason they may ask which trans your running is all in the dollar figure. As we all know manual fto's are harder to find hence more expensive to fix if in an accident.
police don't know anything! I've just completed a garrett gt42 upgrade on an r34 gts-t pulling just over 450hp atw. During a test run we where puuled over for a random check. After the male police officer had searched through the engine compartment, he replied, ahh sorry mate, it all looks standard, but that intercooler is making too much noise ===Canary!
So from this, a manual conversion is something i've been looking at. you can do it yourself, it is a BIG job hence the price tag to fit it. Any quality mechanic can do it, the major part of the conversion is the fitment of the peddle box. The tranny is a simply plug and chug. Although i've had my head and arms inside my fto for the past 8 months, this all seems easy for me. The fto is wonderfully engineering engine, and the NA mods that i've done thus far have helped it by 35%.
Speak to kempy's, last time i checked they had a conversion package for $2k ready to ship.
--Frank