When they say matching numbers on a 340 what am i looking for? My vin on the block or date codes? And where would i look

Small blocks starting with the 1969 model year had the VIN stamped on the passenger side of the block down low. Look for a flat pad right above the oil pan rail.

Sorry for the extra information from earlier. It still may be more than you wanted to know but here's an edited version:
The above comment is correct. What year is your car? A 1969 and up will have the last 8 digits of your VIN stamped on a pad just above the oil pan on the passenger side of your block. However, a complete numbers matching engine wIll have date codes preceding the assembly date of the engine on all the parts from the carb right down to the plug wires and rubber motor mounts. Numbers matching is everything that has a date code. Some date codes of other components rarely may be a little later than the engine assembly date but would have the VIN on it (for example the trans has ocasionally been known to have a later date).

Numbers matching engine block is the VIN stamp on the pad (original of course), nothing more. As for a car being numbers matching, the body number, engine and the trans will all match the VIN (after 68 anyway and some 68s from after January 68).

Thanks for the info..I found that I indeed kept my block from the early 80's at moms garage..and the vin is on it...wished I knew then what I know now..I tossed my convertibles engine out back then also..mopar sold new bare blocks for 100 dollars back then..there was no need to sleeve my original one back then..who knew..

Yea, I bought one of those blocks back in the late 70`s for just a tad over $100. It was still in my volare when I sold it. Later I went back and tried to order another one and they were no longer available.