Everyone is spot on with most likely the manifolds being your issue, however, you may also have a different or even additional problem causing the codes (especially when age and miles on truck are considered) and one thing that would put me over the edge is to spend a ton of cash and have the problem still, when I'm done.
In general, ....The following additional engine related problems are known to generate the PO420/430 code (which means only that your cats aren't being seen by the computer as operating within their efficiency range:
Intake manifold air leaks
Fuel injector problems (leaks)
Incorrect spark plugs
Ignition timing
EGR problem (not an issue with our engine)
Defective catalytic converter
Oil or antifreeze entering exhaust
O2 sensor(s) not operating correctly
Road damage to converter
Silicone contamination
There are exhaust/intake tests that can be used to isolate the issue at cause: Back-pressure test, Vacuum Test, Temperature test. Also:Check the tailpipe, particularly immediately following start-up, for any smoke indicating too rich AFR (black smoke), water/antifreeze (white Smoke), or oil (blue smoke). Inspect intake system for signs of oil indicating excessive blow-by or cracks that may cause leaks. Look at Long Term Fuel Trims (LTFT). These show the amount (in %) the computer is modifying fuel delivery, based on the 02 sensor feedback. Positive numbers indicate the computer is adding fuel, negative numbers indicate the com*puter is removing fuel. Large positive numbers (>10%) should be investigated further as they indicate the computer is adding more fuel than originally designed.
Statistically, across all vehicles with them, O2 sensors are the cause for failed cats most of the time. The most important thing to remember about Oxygen Sensors is that they were never meant to last the life of the car! They are wear items and need to be replaced as you would brakes, tires or spark plugs. When Oxygen Sensors reach 80,000 miles plus...change them. Catalytic Converters do not go bad by themselves, 95% of the time some other issue with the vehicle will destroy the cat and most of the time it is the Oxygen Sensors. When sensors reach that age they may still be functioning but the signal they send becomes lazy and since they operate in millivolts the slightest signal fluctuation can cause an air/fuel mixture problem and that in turn will take out the cat.
So the questions are: do you have the skills and tools, can you find someone with the skills and tools to do the process of elimination that is the root of diagnostics? Good luck with your solution. There aren't any 'easy' and cheap ways to get to the fix, but they are better than an engine replacement!.