If your talking about the crank, the first thing that comes to mind is, what year?
If you go to the Home button, then to Articles, you can peruse the blocks and cranks article. It lists the years that these parts were available.
Next, look at the pad behind the alternator under spark plug #1. The block deck sticks out a bit there. There are some numbers stamped there. Unless the engine has been modified, those numbers will tell you a lot. Sometimes it takes some scrubbing to find out what they say.
Forged crank blocks don't readily accept cast cranks, and vice-versa. It can, and has been done. Chances are, it has not on your engine. Never know though.
We're talking about 225 engines here. The 170 and 198 were all forged cranks.
Which is best? That's a loaded question.
The cast crank is lighter. The forged piece is stronger.
Next you need to determine what your going to do. The amount of strength often gets trumped by the lighter weight for making power. However, there is a break over point where a late crank, like all the cast cranks, will not work with early transmissions without further work. That would be 1967 to 1968. The register size in the end of the crank changed.
A cast crank is still plenty strong for most applications. We've had members here put big power adders on stock cast crank engines and run them for quite some time. To my knowledge, the crank wasn't a direct failure point. Often the level of modification, coupled with extreme abuse, read: Racing, ended with some sort of oiling problem being the culprit.
Being lighter, the cast crank can be spun up faster than the heavier forged part. That can equate to better ET out at the track.
The forged crank will technically handle more engine speed. Due to the stroke and length of the parts, it is generally unadviseable to spin either engine faster than 6000 rpm. While many people do spin them to 6000, wear is greatly increased, and due to varous factors, failures occur. Most stock engines don't run much over 4000 rpm, with a few that live at 5000. This means, for most applications, the forged crank is not necessarily required.
Due to the length of the crank and camshaft, running much faster than 6000 rpm becomes a problem. Enough missalignment and twist can be present to impact valve timing, and cause excessive wear to bearings and bores. An iron block may seem stiff, but at high engine speeds, the block can and will deform. If only slightly, the clearances of bearings can be compromised.
Hows that for an answer?
To improve stiffness, a billet crank can be used. While that will make for a very stiff crank, the long bores can move the block around the crank, so there is still missalignment. Most high rpm engines have shorter crankshafts. Part of the reason V6's became mainstream. A v6 is harder to balance. It can spin much faster, so can make more peak HP.
HP = Torque x RPM/ 5252
If the engine can reliably run at high rpm, the HP will climb accordingly.
The slant six makes good torque at much lower rpm. That's good, because the heads don't flow very well. To spin really fast, flow becomes very important. That's why computing CFM requires that peak engine rpm is important.
There are things that can be done to the head, or power adders can be utilized to artificially overcome the poor flow characteristics of the slant six head.
Have fun with this!
CJ