Finding the foam that is the right density locally might prove to be difficult.

 

I have done my bucket seats and used what they call the foam seat buns. Came out good.
If you don't have experience at doing this I would go for the pre shaped foam. This way you know that the it will be filling out the covers properly. You will be saving money on the labor by doing it yourself so I would get the shaped foam.
Does you bench seat have the armrest? 60/40 type seat or a straight bench?
Patience! Patience! Patience!
If you haven't taken it apart yet pay attention to how things are done. Take pictures of the old seat especially underneath on how the material is attached to the seat frame. Especially the corners where material gets folded over and where any hinges are to let the seat fold forward.
There is a material called whisper cloth that is put on first and allows the material to slide easily over the foam.
Try an upholstery shop for that or use a plastic bag underneath to help. Then just pull the plastic bag out when finished.
If there is any vinyl involved make sure to work in the warmth so it is not hard. I brought mine in the house and done it in my living room.
You will see the pros use a steamer to help get wrinkles out. You will need hog ring pliers and the rings.
Take a look at a company called PUI. They make interiors for car restorations. They had several videos posted on installing seat covers. Don't remember if they showed a bench seat though. But you will get the idea on how to work the material.

 

Thanks for this great reply. I haven't started taking things apart, and probably wont start for a couple months. I'm just trying to gather information before hand.
I suppose the price of shaped foam seat buns is really not that much more that a foam roll at JoAnn's. You right, I'll probably just go ahead and buy the shaped foam.
I'm assuming the bench seat I have is original. Center split with locking backrest.
I saw a few videos of people using plastic bags, but the whisper cloth sounds even better. Is the whisper cloth left in place?
PUI seems to have everything I need which is great!
Did you have to stiffen up your springs are all? Would that be a matter of simply bending them?--assuming they're not broken.

 

whisper cloth can be left in.
PUI will not sell directly to you. They are a vendor supplier only. But very good to deal with. You can all them and they will talk with you. I did. They are very helpful. They will tell you where you can get it though.
I bought my upholstery from Mike Monte's. The foam seat buns I found on Ebay for a really good price.
If you find the seat foam I would grab it. Most of that stuff comes from China. The upholstery is usually made here in the US. That is where PUI is. My upholstery was actually made by PUI and sold by Mikes Montes.
Bucket seats actually do not have springs in them. Just some wiring going across the bottom of the frame.
So don't know what to tell you about that. Check with a local furniture upholstery shop. They could probably help.
I actually did do a bench seat many years ago in a 79 El Camino I had. That is why I suggest the new foam.
I repaired the 79 by adding thin layers of foam and some jute.

 

My wife did our split bench seat. She put foam on it. I'll have to check with her on what density we used. I'll post pictures of the seat what it looks like now. I'll post the density we used and the title of the book we used to guide us.

 

The book my wife used is"Automotive Upholstery Hand book" by Don Taylor. I was mistaken that my wife replaced all the foam. She glued under the cushions that were there.thid is what it looks like up close.