As an older GM guy, and having seen this before, the following: If you have a "mechanic" that is piece mealing it or chasing the problem, you don't need him/her.
Rear mains leak and they start many times with in the first 6 months of being new. You could raise the engine, pull the pan, pull rear cap and replace it, but it is not a do it yourself item when you are first starting to work on cars.
The first thing you learn is that you break more than you fix, and you are not nearly as good as you think you are, and most of the times you will be wrong. It takes time, busted knuckles, lost sockets down the manifold, burnt fingers, smack your head on an engine hoist as you crawl out from under something, pick up and launch your top box across 3 bays of the shop on a bad day. Plus much more way before you get to the point to where you start to think that you might know something.
You have a 40+ year old car, and restorations or even keeping one on the road, is expensive. So the trick is to do the "vital few" first and be frugal.
For anyone working on your car.... "Trust but Verify", and ask questions.
You don't want to have to take out a payday loan to replace your worn out muffler bearing.
So ok.... Old school here. Get a bottle of 5 min flush, put it in... Start engine, run until you hear change in pitch, run no more than 8 min if you don't hear it. Shut down let it cool, drain. Do a full oil change with new filter and new oil, just the cheap stuff. Drive 100 miles and do another oil change. Use high quality like a Wix filter from NAPA, and high quality 10/40W. I always used Quaker State. Add full bottle of Marvel Mystery Oil First, then fill remainder with the oil. Should be 5 quarts total. Don't do 5 quarts and then add MMO.
Put the Marvel in FIRST, and then the oil. This will help clear up the leaking main.
Edit: Forgot to add. After 2nd oil change drive 500 miles and change oil again.
As before..., use a good quality filter and 10/40W. After the 500 mile change, you are back on regular oil change schedule.
Now on the anti freeze leak.... Where is it? This is more important than the dripping oil and your mechanic should have told you this!
The 307 has a front plate that has a gasket to the block and then on the other side is where the water pump mounts to. This plate rusts, and around the holes it can and will erode. Under pressure the coolant will spray or seep out under the gasket.
If you are seeing anti freeze in this area, you run the risk of the coolant getting in the engine oil. From behind the front cover it will drip into the pan and mix with the oil and you can't see this. This will destroy your bearings with the rods being first, and then the mains. Antifreeze has viscosity similar to hot oil but it provides no cushioning effect. This will "wipe out" your bearings.
If the antifreeze (coolant) is from the outside like a tube or hose, or radiator leak, that's not so bad. But if it is collecting low on the block and you can't see where it is coming from, you need to get ready to "Remove and Replace" it is called an "R&R" of the water pump. This is way more important than a few drips over night. Antifreeze in the engine oil will destroy the engine.
The other item is the the freeze plugs, also called expansion plugs. The bad one is on the drivers side front and it is right behind the motor mount very hard to get to. Almost impossible to replace. But can be done.
Find someone that knows how to pressure wash the engine compartment without breaking things and or cracking the exhaust manifolds. The idea here is to start with something clean and then it's much easier to find leaks. Most AutoParts stores have the UV dye and the specialized light you need to use the UV. This is a big help in finding leaks.
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