engine blew, I got it replaced

The two engines look identical. Both are considered "small block" V-8 engines. The smaller one is the very common 318 c.i. and the 5.9L is the 360 c.i.

In the '70s and '80s the engine size was stamped in small numbers under the front edge of the left cylinder head. It would look something like "LA318". "LA" was the engine series, meaning the small block engines. The power steering pump might be in the way, and those numbers were only about 1/8" high so you have to look real close. On the later engines you should find something like "7 M 5.9L xxxxxxxxxx" where the "x"s are the serial number. It's the "5.9L" or "5.2L" you're looking for. The "7" is for the model year.

Also thinking back to the older engines, the 360 was "externally balanced". That meant they couldn't cast all the counterweights onto the crankshaft so they added some to the vibration damper and to the torque converter. Besides looking for evidence of that extra weight on the front, the 318 is not externally balanced and if that engine is used with the torque converter from the 360, there will be an irritating vibration from the engine.

Originally there were four of these engines, going back to the early '60s. The 273 and 318 were the same, and the 340 and 360 were the same. Between them, the driver's side engine mount was just a little different but you just had to relocate one bracket to change from one to the other. That was a two-minute ordeal.

The wiring harnesses are going to be the same between the two engines but the injectors will be different. It is true that someone could have switched the valve covers as they are all the same but that isn't very likely. The definitive answer is to look at the number on the block. I'll draw you a picture shortly.

This shows where the smooth machined area is on the driver's side front of the engine, just under the cylinder head. The engine size will be stamped there.

There's no indication in the service manual as to whether the 360 is still externally balanced, but if it is, you'll find an extra section cast into the vibration damper, as shown in red here. I've never heard of an externally balanced 318 so if you do have that extra weight, you have a 360. If there is no extra weight, you can't say for sure just by that.