turbo 360 on engine dyno see inside!

Just a question..how sick did the car have to get before the leaf springs weren't adequate anymore? I intend to do something similar on my 69 Valiant some day when the economy gets better, on a less grand scale....I'd like to know where the "breaking point" was......you have a video of the Duster on the dyno back in the leaf sprung days, it was FIERCE!
It even broke the tie down strap...!

http://www.youtube.com/user/sdconcepts#p/u/81/2CCmDO3csXc
and
http://www.youtube.com/user/sdconcepts#p/u/80/QhY4b8Qbc1M


BTW, perfect presentation of how a leaf spring suspension is supposed to work.

Thanks,
Alan