The blowers simply DONT flow enough, its not an ENGINE issue, we had one of the fastest KB cars in the country, we've been involved with it for a LONG time. THe ONLY KB car I've ever seen make over 500rwhp was Joe Zeto's car and it was intercooled with a wild twin throttle body intake. The Lightning KB kits flat out SUCK, the only way to make even 400rwhp with them is to switch to mustang manifolds, which is fairly involved.
THe most powerfull KB Lightning I know of is in Cali, its a 427w, AFR heads, a custom cam, aquamist WA injection and 19psi. It made somewhere around 430rwhp.
You cant compare Gen II and Gen I engines at ALL.
I'm well aware of what the 351's will do with a turbo, but a turbo is not a Kenne bell (heat pump). My truck went 13.4@104mph with 6psi and no intercooler on a 2.2 60'.