400 small block on C50 dumper

[Arnulfo]

Going to check the truck out this Friday. Owner used it in his concrete business. 1970 GMC 4 speed with split rear. I am restoring a house and will use it as a dump truck. Owner said original motor died so was replaced with a small block 400. Says he is able to keep up with L A CALI traffic which I find hard to believe. Slow lane averages 75 mph.Your thoughts on this swap? Is the 400 a good choice? I am new to the big truck world. Any advice is much appreciated.