First, in order to identify the block, you must look on either side of the block, Under the heads, in the center near the freeze plugs. Yes this means crawling under the car if the engine isnt out. You will easily be able to see 350. The Letters D or DX are under 350.
If you have ever wondered what the differce is between the D and the DX block, then here is your answer..
The D Blocks where the First 350 Diesels, They are Identical to the DX block except for the casting numbers..They had regular hydrolic cams, these are the ones that Broke the crankshafts as well as other problems in the bottom end..This Block was made up until 1981 when the DX Block took its place. Why was there bottom end problems?..Simply this, The bolts holding the main caps on, had nothing to grab a strong hold of..they where too short. When building a D block engine, I strongly suggest using ARP Main Studs..they will grip all the threads and hold the caps down tight!..
The DX block features a Roller Cam, reducing wear, as well as 1/4" deeper threads and bolts for the Main Caps..which is what made the bottom end problem come to an end pretty much.
The D block isn't a bad block at all..you could always turn it into a DX with the exception of the roller cam (which I am studying carfully), by having deeper threads drilled by a machine shop..other than that, everything else will interchange.
The wrist pins where different sizes..meaning, there are two different wrist pins and bushings.. Engines 1978 through 1982 have 1.050" wrist pins. The 1983-85 motors have a 1.011". Just be sure to order for the correct year to avoid dead ends when building.. I have also been told, The wrist pins from 1978 to 1985 are all the same 1.095. In 1983 they went to a steel backed bushing. 1.200 dia. Which eliminated the bushings from turning. To use this bushing in the 78 to 82 rod you have to bore the rod. I have not yet Compared the Two different types myself..but the bottom line is, when ordering bushings for the rods, specify the year and if possible, take a measurment and have the supplier compare to avoid sending parts back and waiting for the right ones.