Actually, any L36 1995-2005 engine will work. Just transfer over the sensors and your good to go.
It does sound like you have the VERY TYPICAL intake gasket failure. Good thing so far is, its not going into your oil.
Here is your shopping list.
Upper and Lower intake gaskets. Get the aluminum framed lower intake gaskets.
Aluminum coolant elbows. Your going to break them removing the manifolds.
And an oil change kit. New filter, oil etc.
2 gallons of coolant.
Mr WilliamE, our parts research guru should be in shortly to advise the best places to get the parts.
1997 Pontiac Bonneville SEi-SCRAPYARD
2000 Chevrolet Blazer LT 4X4 4.3L
2002 Chevrolet *-10 LS Ext Cab 4X4 5.7 V8