There are kits that tap into freeze plugs in the back of the heads and allow you to install a cross tube. Currently coolant doesn't flow well back there, gets trapped, heats up, and causes excessive wear on the rearmost cylinders (4 and 8). If you aren't beating the hell out of the motor, it is less of an issue, but if you drive it hard, you will cause wear to those cylinders. Several people make kits. Here is more info than you probably wanted....
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4.6 DOHC HEAD COOLING MOD
Introduction to the Even Flow Cooling Mod
This newly designed modification was initially designed around the 4.6 DOHC supercharged application for 2003-2004 Mustang Cobras. This modification is applicable to other higher horsepower 4.6 DOHC engine applications such as those found in 2003-2004 Mach 1s, Marauders, 1999-2001 Cobras, and other chassis combinations that have been fitted with a 4.6 DOHC power plant.
Engine modifications seeking to increase horsepower and torque will also increase the amount of heat generated by the engine. Under ideal circumstances the cooling system as a whole would have adequate capacity to carry excess heat away from the cylinder heads and engine block, keeping cylinder block and head temperatures in a working range that allows proper warm-up, yet prevents excessive temperatures, which lead to pre-ignition, detonation, and overheat conditions. The widespread availability of high flow cylinder heads, superchargers, turbochargers, and nitrous oxide for these modular power plants has allowed modern day street cars to make 2-3 times the horsepower and torque the manufacturer intended. Naturally, excess heat is a by-product of this extra power.
A common issue that has generated significant interest and discussion has been the lack of coolant flow at the rear section of the drivers side cylinder head on Mustang applications (Other chassis applications with a 4.6 DOHC engine may also suffer this potential issue on the passenger head also). From 2002 through the present date, Ford has revised the part number for the driver's side head 3 times for the supercharged Cobras. Many owners and tuners believe that cooling issues, combined with inadequate spark plug threads in the earlier head design, contributed to problems such as bad valve guides, audible ticking noises from the driver's side head, spark plugs getting blown out of the cylinder heads, pistons being overheated, and cylinder bore scoring, sometimes requiring engine replacement. High boost pressures, sustained high-speed runs, and bad tunes contribute substantially to these potential problems.
The passenger side cylinder head (in Mustang applications) has a distinct advantage over the driver's side head. The passenger side head is equipped from the factory with a fitting that allows hot engine coolant at the rear of the head to flow through the heater core and eventually find its way to the back of the water pump for continued circulation. The driver's side head, from the factory, has no such provision. As such, the superheated coolant must find its way back to the front of the cylinder head before being circulated into the radiator. It is believed the factory design allows this superheated coolant to pool near cylinders #7 and #8 too long, keeping those cylinders at a higher temperature and/or a higher rate of temperature gain than the others. To complicate matters further, a vehicle that has the ability to generate a higher level of acceleration force will effectively counteract the movement of coolant from the back of the engine to the front. Is it just a coincidence that a majority of engine problems in power adder applications bein have identified these rear cylinders as most the common?
In an effort to combat this issue aftermarket companies have attempted to use/modify a factory Ford fitting, allowing it to be installed in a rear freeze plug location on the driver's side head. The coolant flow from the driver's side head is then merged with the passenger side head before being routed through the heater core and back to the water pump. On some aftermarket kits, transmission removal is a necessity for installation.
THIS IS THE FIRST AREA WHERE THE EVEN FLOW COOLING MOD DIFFERS. THE INSTALLATION OF THE EVEN FLOW COOLING MOD DOES NOT REQUIRE TRANSMISSION REMOVAL AND IS PERFORMED FROM THE TOP OF THE VEHICLE (BLOWER REMOVAL IS NECESSARY)
Aftermarket companies using the factory Ford/modified fitting tie the driver's side and passenger side head's coolant flow together and then route the coolant through the heater core and back into a metal tube that runs under the intake and terminates at the rear of the water pump for circulation back into the engine (Note: NOT through the radiator)
THIS IS THE SECOND AREA WHERE THE EVEN FLOW KIT DIFFERS IN ITS DESIGN.
The Even Flow Kit creates a completely separate path for the driver's side coolant and routes it directly to the radiator via a custom fabricated aluminum 'T' that installs in the upper radiator hose. The passenger side coolant flow remains in the stock configuration, except for the removal of the INNER restrictor that is located in the inlet hose to the heater core. Once the above modifications are complete, both rear cylinder heads will be flowing coolant through restrictions that are approximately .370-.390 thus maintaining EQUAL coolant flow. Additionally, two completely separate coolant paths are used instead of merging the coolant at the heater core and attempting to double the coolant flow through a single given restriction.
But I got em at Jegs for like $900 shipped new. I looked up another Shelby rim (CS66) and it shows a 6.18 backspace for a 20X9 so it's probably similar.
