I find that using the wrong antifreeze is bad....Using anything other than green isnt good,,
On a 351w I bought for a core on my 408 project the guy removed the engine and cleaned it up pretty good then reassembled it and put it back in he used Orange Antifreeze and only put a few thousand miles on the truck before letting it sit for 4 months due to issues with thetransmssion...When I took the engine apart to do my 408 I took the heads off and a couple of the lil steamholes were clogged with a thick orange gel..
Theres a huge difference between orange and green antifreeze...The orange stuff is acidic and doesnt mix with green well or like being in engines that have green antifreeze residue inside the block and radiator and starts a chemical reaction....
The seal between the timing cover and block develops places for air to enter and water to escape.I find proper torque,indianhead shellac and a retorque after a few heat cycles helps but I also have a fix for that ..
When you get antifreeze in the oil it gets filtered out and eventually clogs the oil filter fast and can be detrimental to your engine..
I took 3/4" copper couplers and epoxied them into the waterpump cover then when I installed the cover I used MotoSeal to seal the tubes where they go into the engine block
The 4 long screws that attaches the waterpump to the engine needs thread sealer as this is another way for air to enter and antifreeze/water to escape..
Another thing I find is that intake manifolds need to be retorqued plus I put a lil Indian Head Shellac around the water ports on the intake gasket to prevent any possible leaks..
Another thing I find is if youre running MLS gaskets is the way you initially treat the engine..I do my initial startups on plain water and even though it says its not needed I do a retorque anyway as my 351w didnt come with aluminum heads from the factory.
Then theres the matter of buying the right headgaskets too...Some manufacturers list if the gasket is for a steel to steel,,aluminum to aluminum or an aluminum to steel application too..........
Usually renting a cooling system pressure tester helps find the leaks fast if there are any..I pump it up to the max pressure my radiator can produce and let it sit overnight with a sheet of cardboard underneath..
If you run the car with plain water or antifreeze thats less than the 50/50 mix and you didnt remove it all during the winter ice could of started a crack in a waterjacket too depending on how cold your winters get.............
I hope you get to the bottom of the problem..