From what the engineers told me when i got my CA done in my 1200, the formula used to determine which engine goes in is done by the heaviest possible version of that car. In my case it was the automatic, fully optioned version of the 1200. Depends on which state you are in of course, i can only vouch for NSW.

I never needed a weigh bridge receipt, otherwise with a full fuel tank and heavier seats etc i wouldnt have made it... :( The reasoning behind it was to limit the amount of influence "outside" paties could have on the situation and whether they could be swayed to say it had "XXX" of weight.