2 questions to start this off -- during the current run of ownership, ( Yes I sold her then bought it back a year later ) my 68 BB seems to run fine with the coolant level 1-2 inches below the mark on the rad, right at 200 on the temp gauge and does not piddle when I get it home. Until last week when it ran fine but dropped a quart on the floor both times out.  Anyone else found this low level thing true? Also, I understand the 68 BB cars had no overflow tank .... what if I have one from a small block car .... would that help???  I know the rad was about 80% when I had it flushed 1st time I owned the car so it may be getting lower .... If I replace the rad, what is the best way to go for a new rad.   --  Alum. direct fit or copper brass, does anyone make an oversize unit that fits without mods? Or will a new orig type with the proper seals really keep it cool? Thanks   

Try Dewitts if you are looking for a replacement. They will tell you exactly what will work best. They are a bit pricey, but well worth it.

The radiator really should never be filled all the way to the top. The system needs a little room for expansion. About 1-2 inches below the neck sounds just about right.
You may indeed have a radiator that is not up to snuff anymore.
The seals around the shroud, and over the top, are pretty important for good cooling, especially on a Rat engined 'Vette.
Do you know if your fan-clutch is operating correctly?

An overflow/recovery tank is a good idea on any car, imo...

Small blocks didn`t have an overflow either for the 68-69`s. I`ve been looking to fit one in on mine. hjowever the ac compressor is in the way. If your car is  not an ac car you can probably fit the small plastic bottle type from an early 70`s car tucked up under the well . good luck

I ordered an aluminum one for my 74 from DeWitts.  I'm happy with the results.  The only suggestion I would make is to go for the "Black Beauty" option (I think that's what it was called).  They paint the radiator black so it looks more stock and it doesn't effect the cooling.

look at an overflow for a 75. mine is tucked behind the front wheel well under the fender.. sticks out just enough to get coolant in.