The actual difference is the heads....the blocks are essentially the same, so check your heads' casting numbers to determine which intake to use. The early heads can be used on the later blocks, and vice-versa.
Yes. The heads are what will determine what manifolds will fit.
As stated above the 273 and 318 take the same manifold. The 273/318 heads are similiar to the 340/360 heads except the larger engines (340/360) have larger ports. The intakes will fit both 273/318/340/360, but the ports will be slightly larger with the 340/360 intake which will be a slight hit on efficiency. A 340/360 manifold will bolt up and work on a 273/318, but will have slightly larger ports.
The LD4B is a great manifold for the 318/273 as the ports will match correclty. When we bought my son's 71 Valiant 4 door a few years back, it came with dual exhaust and an Edelbrock performer intake and holley 4bbl. The Holley carb was in rough shape. We replaced the intake with a LD4B and Edelbrock 625 cfm carb and it ran MUCH BETTER. It ran strong off the line and mid range had nice pull to it, and no hesitation off the line. It's a great manifold for the street.
If you run a cam with 260° to 280° duration, the LD4b with a good Holley 600 vacuum secondary, or Carter/Edelbrock 625, and a nice set of 2 1/4" dual exhaust, you will have a nice running street/strip motor.