Hmmmm, if I didn't have to worry about emmissions and I wanted to beef up an old straight six; this is what I'd do. An engine is not much more than an air pump that makes more power with the more air it moves. So open her up with a better flowing exhaust system. Also a free flowing intake system with a higher duration cam in a ported and polished head. I'd also consider higher compression pistons to make up for the lost compression from the higher duration cam. This combo will make impressive horse power for a six, however there is no substitute for cubic inches when making horse power.