[Bryn Tirion]

Bob.... it would be interesting to see a picture, of your exhaust manifold(s) and of your engine. Your manifold doesn't look like the one in this listing does it?

65 66 67 68 69 Dodge Plymouth Chrysler B C Body 273 318 Exhaust Manifold | eBay

..... which is for a later or LA designation 318. Not the original Poly Wide Block (official designation is as an A engine). This would mean your engine had been swapped for a later LA. The definitive test is to look at the lower edge of your valve (rocker) covers and see if they are scalloped, as in the attached picture. If yes, you have an original-style Poly, if no, you have another engine, more than likely an LA 318.

Edit: I should add that a complicating factor is that the early 313 Bristol engines were really 1960 Plymouth units, and the later 318 Bristols probably had the Dodge Dart unit of 1964 or so, with a four barrel Carter instead of the 2 barrel unit affixed by Dodge. Pictures of your engine could possibly help clear things up. Do you know if your carburettor is a Carter 3131S by any chance?