383 and 3 speed tranny holding up?

[cknfrmr]

I am planning on putting a 383 in my 69 short bed which has the 3 on the tree in it. My brother is helping me with the whole process due to the fact he has all the knowledge on this. He keeps telling that the transmission is gonna go and that I need to put a automatic in it. Am just wondering if anyone out there has run a similar or more aggressive set up and how it held up. I REALLY want to stay with the 3 speed. But also don't want to be breaking stuff either. Thanks for any input or advise.