My advice would be this.
Contact the relevant govt authority in your state and have them provide you with the name/s of engineers certified to sign off on any mods you intend to perform. I would also request from them the latest documents relating to the mods that you want to perform. Having the info supplied by the authority gives you piece of mind and allows you to be informed before discussing the intended mods with an engineer. Try and find someone in the authority that is helpful. Get their name and use them to help you with the info required. If you find that the info is not clear seek clarification before starting on the mods. Get this in writing from the authority. Very important! Sometimes it may seem like a long winded process but it will be worthwhile rather spending money for nothing. Remember at the end of the day it's the authority that signs off the engineers report and if they are not happy your pride and joy will sit idle in the shed :-(
Cheers
Craig