Some thoughts -
1) This may be normal for your radiator and cooling system, so do not assume this is a combustion leak, but you do need to do some further testing since you have doubts. When your radiator heats up and the coolant and water expands it will push coolant into your overflow tank. Then when your system cools it will suck it back into the radiator. So push and pull of coolant is normal, if your coolant tank is low/empty you may see steam going in as you shut off the engine, the coolant perhaps get hotter (Not moving with water pump off.), coolant starts to fill a empty or near empty overflow tank, and you hear girgling (Or other noises.) and see some steam. (An aluminum radiator likely increases this action.)
2) They do sell kits to test for combustion gasses at the radiator cap. Gasses present at the radiator opening then you have a leak from the head gasket or mentioned above from a pinhole leak in the cylinder wall from cavitation. Cavitation is a known issue (Lots of threads on this.), but not an issue I see on this site as a regular problem for owners. (Can happen, but if using the correct coolant, not a very common problem.)
3) If you did have a combustion leak into the cooling system, it should bubble gases with the radiator cap off and looking at the coolant. (Radiator needs to be 98% full) Get a radiator/cooling system tester (Your auto supply may lend or rent you one.) Then when you start the engine the coolant pressure should build and pass 13 PSI pretty quickly as your combustion chamber is pushing 300-450PSI before detination. However head gasket leaks can come and go based on heat and metal expansion. So do not assume one test will confirm. You may need to drive and stop, drive for 20 minutes and stop for 10 minutes, drive up a hill and stop, and then test. You may want to buy the kind of radiator cap with a red tab on the top, that lets your release the pressure before you turn the cap to avoid getting burned by coolant. Always use a cold wet towl when opening a hot radiator cap, the towel with absorb heat (Metal parts or spraying coolant.) and slow the spray of coolant to keep your body safer.
4) Look for signs -
a) Often but not always head gaskets or cavitation will allow coolant to get into your oil and visa versa. So you can pull your oil cap off and look for moisture or milky white oil up inside it. You can drain your oil and look for coolant or water in the drain pan. (But this could also be your oil cooler or water pump bolts, so again careful to not just assume its the head gasket or cavitation.)
b) Often coolant leaks out of the head gasket to sides of engine block. (Can leak to the outside or to the inside!)
5) Remember to head the warnings that members of this site have all shared about the cooling system.
a) Use coolant with SCA (Like Fleet Charge) and manage that. (Use strips or via coolant changes.)
b) If the coolant is not already diluted, dilute to 50/50 only with distilled water. (Or risk suffering from rust and failing seals like head gaskets.)
c) Use only the Motorcraft or IH thermostats, too many posts of issues with others.
d) When replacing the water pump remember some bolts go through to the cooling chamber, so use sealant on those.
e) Keep your IP timing in the zone, new IP, new injectors, or just a new rig. Check the timing it can effect the temp of your engine.
f) Use a good radiator cap rated at 13 PSI. (Maybe less if you always ran say 8lb one, raising the PSI this late in the game could cause a new leak.)
g) Have a good coolant overflow tank, it can help you understand what is going on. (The level is changing fast to empty, you likely have a coolant leak.)
So do a few more checks to determine if you have an issue. Then worse case rent a car for your trip, then you will have peace of mind and not get into a bind if you break down.
All the best!
