5.9

IMO, yes, good engines.

Now the /6 to V8 issue is easy, but does take some time to get the parts together which can be a bit of $$!.

Depending on the year car, trans Dapt has inexpensive mounts that use the early biscuit style. Creative engine services .com (IIRC) has swap kits. Mancini racing also sells them.

Then you have to get a trans for the V8 small block. You can not use the /6 or a big block trans. Then cut your drive shaft to fit. Or find one that fits the car, trans used and rear size.

In the engine bay, you will have to lengthen and shorten some wires. (No big deal right!)
The radiator should be swapped out for a V8 radiator. You'll be lucky to use the /6 radiator with success. And all the hoses that go with it.

IMO, an upgrade is rear springs and T bars are an excellent idea for street driven rides. The /6 suspension do OK at best. And then the brakes.

It can get a bit pricey. I did it but fade to keep the cost in my head. I used a 318/904 which was free. The rear 8-1/4 was $200 and the. I had to add everything on it except backing plates. The front remained drums. The rear springs I got from ESPO.com. They were a 1 inch over bend for a '74 Duster. The driveshaft was purchased used here IIRC.

After that, wheels, tires, electric fan and engine bolt in parts. Intake, headers, (purchased here at a good price.) Jegs exhaust kit @ 2-1/2 inches and a small Crane cam & a MP distributor. Also bucket seats, B&M shifter and Lokar cables. Small items are not listed like band clamps and gasket sealent, etc....

It was a fun car that sounded good and ran fine. Make a list (mine as a guide perhaps) and tally the cost. That should help you decide if the cost is worth the V8 rumble and fun.
It was for me.