Mopar engines are divided into several groups:
The slant 6 : 170 and 198 cubic inch are known as "G" engines
The slant 6 : 225 cubic inch is known as the "RG" engines.
The "wide" block 318 cubic inch (1958 to 1966) is known as the "A" engine.
The small block 273, 318, 340 and the 360 cubic inch are known as the "LA" engines.
The 361, 383 and the 400 cubic inch engines are known as "B" engines.
The 413, 426 wedge and the 440 cubic inch engines are known as "RB" engines (raised block)
The 426 cubic inch Hemi is in a class by itself.

To locate your casting number, look on the side of your block and compare it to the chart
below for engine Id, the date may also be stamped on the block by the casting numbers.

There is also the id stamp on the engines:
On the slant six engines it is located just below the
cylinder head on the passenger side of the block.
On the "A" and the "LA" engines it is stamped on the
drivers side front of the block just below the cylinder head.
On the "B" engines it is stamped on the top of the block near the distributor.
On the "RB" engines it is stamped on top of the block
forward of the intake manifold on the drivers side.
The stamps may include the engine size and also a series number for the year.

A=1965...B=1966...C=1967...D=1968...E=1969...F=1970...G=1971...H=1972...J=1973...4?=1974

There could also be a stamping "HP" or "HP2" designating High performance.