The engine swap i'd be tempted to do would be the Buick V6 turbo into the catera...you know, the Buick T-type or Grand National engine.....might not be toooo much faster than a stock catera, but do you have any idea what the aftermarket is for those engines?
Plus, its a v6 which is lighter and smaller than the V8...
If your going to do a V8 i would go with the LS1/t56....theyre on eBay alll the time and you could get a decent one for 3,500-4,000 dollars probably....
Theyre small and lighter and more powerful than the northstar probably.
Only downside to the LS1 is the avalibilty of aftermarket parts, and the price. If you were to do an LT1 swap, your options as far as upgrades would be unlimited....the Chevy smallblock 350 is probably the most widely used V8, and people have been making aftermarket parts for them for decades. for probably a thousand bucks less than the LS1/T56 combo, you could get an LT1/T56. Then with the extra money, if you want power, save up for the LT4 converson kit......proven 425+ HP. I've also seen 383 stroker kits on eBay, that would bring STOCK hp up to 430 or so HP plus gobs of torque.....and on ebay they go for under 700 bucks. By doing both of those , you'd have a completly rebuilt motor, probably like 500 or more RWHP, and so many inexpensive aftermarket parts avalable should you ever decide to change anything. That would be a MONSTER.
If you have some $$ and just want some power, why not see about a custom supercharger or turbo setup?
You could pull the turbos off like a Saab or something for cheap, just get the exhaust made, some brackets and other hardware, and instantly have more HP and torque.
Have you given any thought to putting a 3.6 liter CTS engine/transmission into the CAT? I'm sure that the Auto recyclers have some on the racks. More power, and no rubber band timing belts.And you can get the computer modules too!