When I bought my 69 the seller told me he was including the original engine to the car. The date code is 8149. Beside it there is a pointer pointed at D while other choices would be N or 5. The block casting number is 9790071 which appears to be correct for a 69 400. How can I tell if this engine is in fact original to the car?

Check out list of engines with the parms you gave me.

I think there should be a number stamped on the front left(drivers side) on the block just below the head that matches the VIN of it’s original. It may be partially hidden by the radiator hose...

The VIN stamped on the lower left front will ice it. The two letter code on the front will be either WZ, WQ, YT, or YW for a 400 in a 69 bird. WQ and YW are Ram Air III

https://firstgenfirebird.org/1969-info/1969-technical-information/1969-id-numbers/

The date code is Aug 14, 1969

Sure would be nice to hear back from people as to what they found, his block number is correct for that year but the real tell is going to be the block and head codes

Here is how you verify your engine is the correct engine for your 68 Firebird.

1. Order your PMS docs from PHS-Online.com

2, On the "Car Billing History" you will see the VEHICLE IDENTIFICATION NO. (VIN) in upper right hand corner. Within the VIN no. is the "SERIAL NO.".
This no. represents the sequential no. assigned by Pontiac. You use this no. to verify you have the correct body for your car. This no. is stamped on top of the Upper Cowl panel. It is also stamped on the cowl under the heater fan. These are the HIDDEN VIN's

3. In the center of the page you will see the "ENGINE UNIT NO."
This no. corresponds to the number stamped on the block under the 2 letter code (engine code) on the RH side of the water pump on the block.

4. Typically you can determine what engine you have fairly quickly. Read off the 2 letter code off the block and read the code off the center manifold head. Go to Wallace Engineering and query the engine & head codes:

http://www.wallaceracing.com/enginesearch4.htm