Yep ..they make adapter plates to go from a small block to a LS..that's what I used on my 6.0 LS 81 truck..you could use those but if it was mine i would rather redo the mounts to purpose fit a LS if you go that route. You can do a crate LS engine way way cheaper than that one you have a pic of. For instance.. here is a crate 580hp supercharged for just over 10gs..I know you said you were shooting for 700 but I can tell you this and feel confident about it...580hp will destroy your tires just as easy as 700hp..just not much weight in these trucks to make em hook. Another thing you might want to think about is the torque limit of a t56 before you shoot for 700hp...
http://www.gmperformancemotor.com/parts/19331507.html