72 Demon stock weight ?

Hi guys , my friend justy got a 72 Demon for a good price and is alredy thinking in testing the car at the track before making motor ,transmision ,rear upgrades .

I dont know nothing about Mopar motors but I really like this cars looks and motor sounds.
He told me he also got a 400 motor with a 927 transmision to upgrade it .

It really a 400 , how to tell ?

Thanks , I only want to help him get the best performance with your help .