I'm no part number guru, but I've been down the same road as you and you've almost for sure got a 413/426 wedge/440 distributor. Those are about 1" longer than the 361/383/400 distributors and look otherwise identical.

You have another option though. Years ago I switched to electronic ignition and I had a 383 in the car at the time. I knew I was going to drop a 440 in it (motor 1/2 built). I found a Direct Connection adapter to make the 440 distributor work in my 383. It was an aluminum spacer about 1" thick that fit directly under the distributor and above the block. It also came with a longer distributor hold-down. It fit/worked like a charm. I probably still have it somewhere.

Hey, I found it at Mancini for $65
http://www.manciniracing.com/rbtobbldiad.html

70 Roadrunner convt. street car 440+6, NOS, 4-spd, SS springs '96 Mustang GT convt. street car '04 4.6 SOHC, NOS, auto, lowered "Officer, that button is for short on-ramps"