You'll be money, time, and effort ahead to just buy a GT. The pushrod 5.0 based engines were never factory installed in the '05+ chassis, so everything from motor mounts, fuel system, trans, and especially electronics will have to be a "figure it out as you go" process, with no factory model to follow or ready to go parts to help any of it bolt in and/or function easily. As a basic rule, that type of project is always longer, more complicated, and more expensive than is ever planned for. On top of that, if you live in an emmissions testing zone, the end result of such a project will never pass visual inspection, and it's iffy it would even pass just a sniffer test. Let's say you do it anyway, and successfully get a running, driving vehicle out of it. A year or 2 goes by and you decide to sell it to fund another project. You'd be lucky to get 1/2 of what you invested in it since most buyers will see it as a backwards teched, bastardized car, which basically would be correct since the mod motor is decades newer in design than the 5.0 pushrod engine.
From a practicality issue, there really isn't any point to it either. Comparing stock to stock, the '05+ 4.6 3V in the GT makes 300 HP vs 215 HP for '94-'95 5.0's. Of course you can build the pushrod engine with aftermarket heads, intake and cam and get close to 350 HP, however you can just as easily get that level of power out of the 3V 4.6, and it will be smoother and more efficient while doing it too. The 306 will add a fair amount of weight to the front of the car as well. The 3V 4.6 is an all aluminum engine, and is pretty light for the power it makes. The iron 5.0 block, even when topped with aluminum heads, is still a bit heavier.
If you're just looking for a late model chassis to build an "off road only" race car, then sure, just get a cheap high mileage V6 car, strip it down to a rolling chassis, weld up some mounts and stuff it with what ever drivetrain floats your boat but if you want a reliable street driver that retains the function of factory features like VATS and OBD II engine management then it's 100x easier to stick with the mod engine family.
