There are two different types of radiator surge tanks. There are the type that are open to atmosphere and aren't designed to hold pressure and there are the type that will hold pressure and use a standard radiator cap.
The type that can hold pressure can be tied right into the existing cooling system anywhere. But generally speaking, you'll want to tie in at a high point so air can bleed up and out of the system into the surge tank (this will help keep the system properly purged). Yes you can tie into a heater line or radiator hose via a tee fitting. It would be best to tie into either the heater core return line or radiator hose return line so you are not on the pressurized (by water pump flow) half of the system.
If you want to use the non-pressure holding surge tank, you will need to get one of those in-line radiator cap fill points that generally install into the upper radiator hose. There should be a 3/8" nipple on this fill point for you to hook up to your surge tank. The problem with using something like this on a 3800 is it can be difficult to fill the system from the upper hose because of the thermostat being present blocking coolant flow into the engine from that point (if using a stock 3800 t-stat housing).
Having said all that, I would still run the stock surge tank up front at the radiator because it can help purge air bubbles from the high point at the radiator. For what it sounds like you are wanting to do, I would run the stock surge tank up front and a pressurized type in the rear by the engine. But, in truth, you don't need a surge tank in the rear. You can simply install a fill point at the highest point you can find on the engine side and cap it off when it is full and you shouldn't have any issues once the system has been completely purged of air pockets.
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