s10 is most popular. https://www.jalopyjournal.com/forum...-rearend-in-my-52-chevrolet-styleline.396656/
You can use a 1970-81 Camaro/Firebird rear. (55 1/4 " wide) I used a 1968-75 Nova rear on my 50. (54 1/4 " wide) But I'm running 10" wide rear wheels. Either set up requires new spring perches to locate the rear on the springs correctly.
The '51 leaf springs have an offset spring pin, so you'll need custom blocks to center the wheel in the wheelwell when using a later rear axle.
I have installed 66 -67 Nova , Maverick , 57-59 Ford , 55-57 chev was probably easiest . My choice today Ranger 8.8
TCI makes a rear spring kit that mounts the axles where they belong. The springs are a little wider and you can then add traction bars or Cal-tracks if you want more traction. The early Camaro and 2nd Gen Nova are the correct width. Axle flange to axle flange, my 51 is 60 inches. My 10 bolt I originally was going to put in measured the same out of a 68 Camaro. The 9 inch I'm now installing is a little narrower to put more tire under the rear. Pick what ever rear end works for your future plans. The GM 10 bolts work for most moderate hot rodding. A 12 bolt GM can handle more power. The S10 would be a happy medium and a Ford 9 inch will handle almost anything you can throw at it if it's built right. What are your plans for the car?
Before you buy ANYTHING............. Get the tires and wheels you plan to use and set them under your fenders so you have about an inch clearance from each fender. THEN measure the distance between the inside mounting surfaces of the wheels. Tire sizes vary and wheel widths and wheel offsets vary. You need to have the wheels you want to use before you buy any rear end. Get that dimension and then we can give you a better choice for what you need. Not doing this is why you see big gaps between fenders and tires. Most likely, an S10 rear will give you that unsightly gap. Worse yet is finding your rear end of choice is too wide........ The only correct way is to get your tires and wheels and find out what width will make them sit right. Next, what type of engine and transmission will you use.....both now and maybe later on. If its gonna have much horsepower, then that's a consideration as well. There are lots of choices, but if we have the right info we can probably help you find a good choice.