Just a little bit of info. The 4T80-E has alot of torque management, its numbers may be above a Series 2 3800 Supercharged (L67), but it actually does worse as far as actual track numbers go. If you are into modding and really want to go fast with a bonneville, you are best off with an SSEi. With the help of Intense, you can exceed the power of an LS2 and a Northstar with EASE.
In short, the Cadillac Northstar is a good engine for cruising, and has VERY good top end, the 3800 on the other hand has insane low end power, therefore making VERY quick track runs easy as pie.
There is close to no aftermarket for a northstar as well.
Some other blown members will further educate you on this matter, but, personally I recommend the 3800 for modding. It also has iron block and heads which gives you the peace of mind of not stripping out anything when rebuilding.