Though rare an L78 is a useless engine in a big heavy car. A 396 is too small of an engine for a heavy car (which is why Cadillac had a 472 cube motor under the hood back when your car was new). Second thing is the heads were great for running all day at 7,200 RPM (assuming it didn't eat a valve), but below 4,800 RPM the velocity was so low in the ports that the gas fell out of the air stream, which didn't burn.
You can use those heads assuming you find another L78 but you need a 496 to a 502 cube motor under them to drive on the street. This is why Chevy went smaller and smaller on port size on the 454 until they reached the same size port as is used on the biggest head for a production .SBC (190 cc intake volume).
The second thing wrong with those heads is they are the original 1961 "bath tub" design. The small combustion chamber is great for building static compression. The L-78 only runs on race gas today as pump gas: is today's premium is 11 octane numbers bellow the old regular gas was from 1966 and the L78 and L88 required 104 premium leaded 1966 gas. Those "bath tub" combustion clambers (called closed chambers) shrouds the valves preventing deep breathing to match the port size of the head. In 1969 Chevy offered "Open Chambered" heads, to allow the valves to flow freely asa Service Part ((they became a production part in 1971). Speaking of shrouding the valves; a 396 has such a small bore that the block had to be notched to keep the intake valve from hitting the block. And that was with the small 2.19" intake valve diameter.
The poor combustion chamber design, the small bore and the small displacement makes the 396 only good for scrap metal. That is were I put every 396 I found back when I had my shop and was building cars. A 400 cube small block Chevy will blow away any 396 you can build and it is 220 pounds lighter than the big block. If you want to replace your L78 buy a 502 short block or build a 496 out of a Gen 6 454 and put on aftermarket aluminum heads with 2.30" intake and 1.88" exhaust valves with a hydraulic roller cam around 236 degrees of duration , and of course the L78 396 decal.
Well I kept the heads off of them; but yes I scrapped every 396 that came my way because a 427 ran noticeably better and I was building street cars for other racers as well as myself. I never raced on the street myself as I had a key to the front gate of the local drag strip (thanks to a neighbor of mine by the name of Don Garlits who tested there when it was closed).
As far as that goes, before everybody and his kid uncle moved to Florida there used to be five quarter mile long drag strips within a forty minute drive of my house back in 1965. All are now built over with home sites except Bradenton which remains an NHRA certified track. The track that was three miles from my house in Sefner that I had the key to was covered by a half mile long warehouse that was the birth place of Rooms to Go. You can see it from space on Google Earth located at the intersection of SR 579 and I-4.