IMHO, the numbers matching engine will only be worthwhile if you're planning on keeping EVERYTHING original and bone stock. Otherwise, even with the numbers matching engine, its not a true OE restoration. I personally don't think the transmission is a huge deal even if you go back and get the original engine and do the full OE resto, date code will be close enough for the transmission for most folks.
But, if you are going to go the OE route, you'd better have the fender tag and all the other goodies to go with it. No point in getting the numbers engine if you're going to stray with any of the other details.
If the guy with the engine is just building a 408, why not offer to trade him the 340 short block you already have? Seems like if he isn't interested in a deal that takes care of everything he needed the engine for he's going to be a problem, trying to get rich off your desire to stay original.
Stuff like this is why I prefer not to mess with OE resto's or numbers matching anything. /6, 318, and non numbers cars are so much easier.