There is a little bit to it.
Method One:
Easiest is to just get the Aussie 200 cube crank from 1970 to 1984 Aussie Falcons. These were large main bearing cranks which sat in a block much the same as the US 250. The rear crank bolt PCD (pitch centre diameter) is exactly the same as the US 250, AT 3.00".
After 1984, the Aussie 200/3.3 cranks got different rope seals (became a neoprene seal), then a latter type in 1987, the so-called Dog Turd (see posts by backlash).
With the Aussie 200 and 250, either early Log or later cross-flow, the thrust bearing width is differnent (alloy dave and Jack have noted this in there posts) and there may be small changes to the front crank snout. This would require the least machining. With a set of 6.27" rods which were standard, you have a 2:1 rod ratio, a 33% improvement. But it comes at a huge price. Loosing 50 cubes is pretty bad.
Method Two:
Jacks suggestion, the 200 US crank . This have three very important points to consider
Part A:If you do a search, what Jack says about bearings is backed up by Aussie7Mains. This is the Grumpy Jenkins approach, which uses a set of 200 bearings held to the crank by a 1/8" roll pin tapped into the orginal 250 bearing. (The roll pin sits below the small 200 bearing surface, it mearly helps peg it form spining!). The tin coating on the bigger 250 bearing is cleaned off, and the bigger 250 bearings form a solid mounting point for the 200 bearings, if everything is sized right. Spacers could be made up to duplicate the 250 bearing. It seams that the smaller one goes on the bigger one like a dove-tail.
The crank snouts are similar, but it needs to be checked.
Pert B:The crank flange on 144-170-188-200-221's is about 3.3", the post 1970 Aussie 200/250's are 3.6". The crank flange is very, very different in size. The rear main cap bolts, in fact all of the main cap bolts, are spaced much further appart. So there ae two options. The easiest is to weld the crank up at the rear main seal with a uniform 200 thou of long run downhand continuous weld. Then it needs to be machined to suit.
Most rope seal cranks have a sharp edge which is larger than the machined surface which is hard to duplicate, but it can be done. Note that the PCD is 2.75", so you'll have to look around for a Big Bell 200 157 or 164 teeth flexplate to suit the smaller pitch. Or risk redrilling the plate by slotting or advancing a new set of holes 30 degrees further around. THIS IS A RISK THAT NEEDS TO BE CAREFULLY CONSIDERED. All Ford I6's carry a nice flange, which will stop the flexiplate or flywheel taking off for your right ankle and removing it form your body, but you never know...
Part C: All 144-170-188-200-221 cranks can get a custom made seventh bearing carrier/ rope seal housing made. It is rather complicated. What you do is get both blocks, and copy the 200 detail into the 250 setting. It is expensive as the machining must result in a perfect match from the bigger 250 block to the smaller 200 crank.
Expensive, but it saves the danger of the welded knife-edges rope seal slinger geeting snapped off.
Method Three
I did my 221 crank (a 3.46 stroke 200 crank) into my 250 X-flow block. I had the main bearing crank welded up to the 250 size, and the fillet radius turned to mate up to the cast iron on the crank counterweights. The thrust bearing is the index point for the crank, and it needs to be prefectly placed. When the main bearing is enlarged, the relative postion of the new 250 blocks thrust bearing needs to ensure the donar crank sits exactly in the designated position.
Then I had an engineer machine up the custom seventh bearing carrier/ rope seal housing. It isn't sealing correctly, and the crank tunnel can't be line-bored untill it is. But the details are right on. This retains the existing 221 (same as 200) rope seal housing on the crank, with its 3.3" flange.
Hope this helps get it clear.
Sorry, no pictures as yet.
A LONG footnote. Not telling you a 200 is a bad idea. The loss of 50 cubes is, though. Any L/R (rod Lengh divided by stroke ratio) improvement is good in itself, but it is a major money sapper. Pistons, rods, and often crank. Destroking is never worth it unless it is balanced against the performance benfit and costs.
In Indy racing, the Cosworth DFX became a perfect engine when it was turbo charged and destroked to 2.65 liters, from 3.0 litres. The L/r ratio was about 2.2:1! All alloy engines need this to reduce thrust loads, especially at 80 psi absolute boost!
I'm using 6.27" 200 Aussie rods, 305 Chev pistons shaved to 1.475", and this gives me a 1.81:1 rod ratio. It is my opinion that 1.8:1 is the best L/R ratio for a cast iron block engine. At 2:1, a cast iron engine is very heavy for its capacity. At 1.5:1, it is very light for its capacity. 1.8:1 is about the median, and I'd strongly suggest that you use the 221 crank.
My 228 is 22 cubes down on the 250. I could have gotten a better result just buying the ACL Race 1.163" pistons with 6.27" rods from a 3.3, with NO CAPACITY LOSS! L/R ratio is 1.60:1, much better than 1.50:1. No capacity loss. ACL cast pistons can hack 500 hp of boosted turbo engine, with no failure. And they are dimemsionally more sound and much better cost wise than a set of Ross, KB or TRW forged pistons which have to be custom fitted to the 250 six. They mostly are designed for 215/3.9/4.6 Buick-Olds-Rover V8's, or 229/305 Chevy v6's and V8's. The stoke sizes for these require a 20 to 56 thou bore-out of the existing block. If you are turboing it to a high level of boost, some ACL's may offer more cylinder wall support at 3.68 to3.71 (0 or 30 over).
Forged pistons do run cooler, though.
Whatever, I hope this is what you need.
