You can always do compression and leak-down tests, actually for such high mileage those are really good idea. Pressurize the cooling system for leaks, also test the coolant for SCAs. Send a sample of your used engine oil to Blackstone Labs for analysis, if you can afford it.
As for your plans - good luck with the 4" true duals, your front tank is in the way of the driver-side piping. Oh yah, tis injection pump, not ejection pump