The 426 hemi is TOTALLY different from any of the early hemis. In fact, Chrysler Corp. threw out all of the design and engineering from the early engines shortly after discontinuing them in '58 so they had to start with a blank sheet of paper when designing the 426 in '63.
that being said, all of the early engines were very high quality machines. All had forged rods and cranks, hi-nickel iron castings and superb machining.
Unfortunately, Chrysler, Dodge and Desoto all made different series engines and none of the parts are interchangeable between the marques.
Chrysler's models were;
1951 - 53 331 long bellhousing
1954 - 55 331 short bellhousing
1956 - 57 354
1957 - 58 392
Several industrial and marine models
Dodge Models were;
1953 - 54 241
1955 - 56 270
1956 315
1957 - 58 325
Various industrial and marine engines
Desoto models were;
1952 - 54 276
1955 291
1956 330
1956 - 57 341
1957 345
I don't think Desotos were ever used in industrial and marine apps. Not sure of that though.
Dodge trucks had all types of Chrysler, Dodge and Desoto engines over the years.
Although the relationship of these engines was unmistakable due to identical construction features, nothing interchanges between the makes.
I don't know about interchangeability within Dodge and Desoto but within Chrysler a lot of stuff is interchangeable. For example any 331/354/392 valve cover and or rocker arm assembly will fit any other engine. 331 & 354 engines are virtually the same except for bore so cranks rods and pistons (with a 1/8" bore job on a 331) are interchangeable. Heads are interchangeable on all the engines. In fact, 331 heads are popular with the 392 & 354 drag racers because they have shorter, better flowing ports. If that switch is made, 1/2" thick spacer plates must be used to fit the intake manifold since the 'big block' 392 has taller deck than the other two 'small blocks'. Cams are interchangeable between the 331 and 354 and although they fit in the 392, lobe displacement angles are different due to the taller deck so the cams won't run well.
The '51 - '53 long bellhousing Chrysler 331 is probably the least desirable style because of the obvious problem in adapting transmissions but also due to the heads having smaller valves and small round exhaust ports whereas all later engines had larger oval ports. However, Don Garlits used one of these in his first rail top fuel dragster so they still can be made to put out power.
the Chryslers have a bunch of speed and rebuild parts made for them. It is fairly easy to build and hop up one of those puppies. Not so for the Dodge and Desoto. They had a modest amount of speed equipment made for them back in the day but it is rare and nothing is currently in production for them. I have heard rumors that Hot Heads is tooling up to make some new hop up parts for them. I don't know why but Chrysler engines of all sizes are still readily available but the Dodge and Desoto engines seem quite rare.
As far as potential, they are still among the elite engines for all-out power potential. None of their contemporaries from other car makers is even close. They can, and do in the nostalgia racing circuit, compete heads up with any modern big block engine.
Here are a few sites to visit.
The Early Chrysler Hemi Registry
Hotrods and Hemis. This is the home page of our illustrious HR.com member and moderator Centerline!
Imperial Club
Hot Heads. These guys specialize in early hemi parts.