What's the best way to connect a Lionel prewar and postwar car?

They make a conversion couple. ( have seen them on Ebay but would check with Jeff Kane at the train tender also jeff@ttender.com ) Some also will take a prewar car and remove the couple from one end and add a postwar couple the type you connect to the axles. I have also seen with box couplers where they added the postwar so it cam out the other side and you can have both incase you want to pull some prewar from that car

I have a story about the very same issue.  Way back in the '50s, my parents bought my brother and I a used set or should I say collection of Lionels.  It came with a 4 x 12 platform, two loops of track, buildings, signals, etc.  Two complete sets of trains were also included.  Both pre-war "O" gauge.  One was a pre-war steam switcher with three madison cars.  The other was a gun-metal steamer with three red and silver tin passenger cars.  The latter set had the hand operated silver couplings.  I know there's a name for them.  The madison cars and switcher had the box type couplers with small hook.  

Long story short, the switcher wouldn't run.  So the next year my father bought a 2065 type steamer and tender with knuckle couplers.  Now, how to couple the madison cars to the new loco.  At first I used a piece of string.  Remember, I'm about ten years old.  The following year, I brought one of the Madison cars to S&H hardware on Castor Ave. in Northeast Philly.  When I got the car back, they had removed the six wheel tin truck and replaced it with a 2500 series four wheel truck with knuckle coupler.  

Now the car sat low on one end and I couldn't get used to looking at the different truck.  So a few weeks later i went back to S&H and asked if they still had my six wheel truck.  I cannot recall if they made me pay for it or not, but I had it now and did some of my own surgery to mate the knuckle coupler with the old truck. 

My point in telling this story is that sometimes Lionel service centers were not as committed as they should have been.  I guess they figured that dealing with a ten year old they could get away with a quick fix.  

Many years later, 1980 I had an MPC GG1, from 1977.  It wasn't running the way I thought it should so remembering S&H Hardware from my youth, I figured I would take it there.  And so I did.  Weeks went by with no call from them.  Finally after about three months I called them.  They still hadn't looked at the loco, the clerk told me.  I said I would be in to pick it up.  When I went to pick it up they wanted to charge me.  I said you told me that you haven't looked at it yet.  After some words, the clerk said here, take it, there's no charge.  When I got home and opened the box, the loco was in pieces.  So they didn't look at it, did they ???

I don't know if S&H Hardware is still in business.  If they are I would not recommend them.

  I didn't have part numbers and was in a shop that wanted me to buy a Greenberg book and look them up myself. I wasn't opposed to looking them up, but wasn't about to buy the book to do it when a shop book was sitting two feet away. His kid caught me on the way out and I delt with him from then on in.

The "transition car" with different couplers is an old trick, though it can be a toss up on "level" or a smooth transition of rooflines, and the body to truck ride height difference too.  Look for something looking similar in truck type and height. 

I have a Marx "Tilt V"  El Captain car with a Lionel truck on one end so I can pull them with cast Lionel. At one point I had a rather unsightly dual coupler truck. A "spinner" set up.

I have a "spinner" tender with both drawbar reciever and a coupler, and I can spin it for engine drawbars with the coupler tucked under, and facing the rear. Or with it spun so the coupler is fwd. it is for use as an auxiliary tender or movent by a diesel or dockside loco.

Open latching and Scout Couplers is how we refered to the early ones. The Scouts having the cast housing / buffer looking similar to some protypical pin & link coupler's housings, but like the "lobster claw", very large versions of them.

I have used twist ties with O and Standard gauge. Easy, quick, simple although I do have  coupler adapters I made plus several "transition" cars plus a box cab engine.  Several ways to skin a cat.

Prewar engine, postwar tender.

Get some Harbor Freight rope and have a lashup!  Just kidding.