Casting numbers for most all Mopar blocks here:
http://www.rosevillemoparts.com/reference_castingnumbers1.php
>casting # 2806030-273-1
>stamped # PM273R 25392602
First number is probably 2806030 Which is a 273 made between 1966-69, LA Engine
Since you've got a '65, that seems like maybe a replacement engine.
What does the Fender tag say? Was it originally a 273 car?
(I don't think the blocks included the VIN information until 1968)
If you check the heads, maybe you can see if their date code is pre-66:
http://www.cowtownmopars.com/original/CastingNumbers.html
Small Block Mopar Head Casting Numbers
Casting # Chamber Vol. Intake Valve Exhaust Valve Comments
2532080 1.78 1.5 1964-65 273
2536178 1.78 1.5 1966 273 Early
2658234 1.78 1.5 1966 273 Late
Here's another chart snippet with the earlier 273 casting numbers - but it shows your casting number as going from '65-'69, so maybe it's original?
http://www.cuda-challenger.com/cc/index.php?topic=2776.0
Small Bock Imformation
Cylinder Blocks
Casting Numbers Engine Years
2465330 273 64-66
2536130 273 65
2806130 273 65-69
If it's still got the original vibration damper, maybe it will shed some light?
Vibration Dampers
Year Engine Part # Crank Type
64-65 273 1947235 forged
66-69 273 2658458 forged
Also, head casting numbers here:
Cylinder Heads
Year Engine Service Part # Casting # Combustion Chamber Volume (cc’s)
64-65 273 2532080 2465315 57.0-64.5
66 273 2536176 2536178 57.0-64.5
67 273/318 2806213 2658920 57.0-64.5
68-71 273/318 2843674 2843675 60.0-67.5
Another way to try to tell if it's a '65 or '66 engine - the intake manifold ports are different
The angle of the intake manifold bolts are different '64-'65. '66 and up match the 318. I don't know if the manifold gaskets are different, but if you found a 318 intake and it wouldn't bolt up, that would indicate you've probably got a pre-66 motor (or more likely, that you have pre-66 heads)?
(Note: found this reference re: intake 'not' working: The difference is in the intake bolt holes, they are smaller and at a slightly different angle. Many people have used a 66-up intake with washers on the bolts. You could also swap to 66-up heads.)
Hope that's somewhat helpful, and welcome to FABO!!
