StrangeRangers quotes are totally spot on. Drag-200-stang used a 200 Ford crank in a US 250 block in his turbo pro-streeter, so you'd be advised to check his posts. He used alloy rods, and it made all the power you'd ever want... 450 hp or so.
Mathematically, from four independent sources, the optimum cubic inch for a Ford I6 is a 56 thou over bored 250, destroked to 227 cubic inches, and a 1.8:1 rod ratio. Rule of thumb is that whenever you improve the rod ratio, you get half the improvement in power. Confirmed by Stan's TRD Formula Toyota engine, Engine Analyser programs, Ford Australias X-flow 200 veres 250 figures, and David Vizards 1435 veres 1559 cc dyno engine, where this very factor was examined in the early 1980's. Frictional losses due to side thrust component were calculated on a Super Flow dyno.
Basically, if it could be done, a 19% improvement in rod ratio on just a 250 engine would yield a 9.5% pro rata power benefit, and a lessor percentage in torque.
In practice, even a shorter piston shorter than 0.875" compression height limits the best rod ratio increase to less than 1.65:1. The shallowest commerical I6 piston is the 1.04" Iapel or Wiseco blank. With some kind of 6.475" costom rod, 1.65:1 is it, and the power gain is half the 9% improvement in rod ratio. So your 250 would behave like an engine 4.5% bigger, or 11 cubic inches better for power.
On the Aussie 200, which is a destroked 250, loosing 50 cubic inches is offest by a rod lenght increase from 5.88 to 6.27"...this improves the rod ratio by about 34%. The specific power per cubic inch goes up 17%. A stock 250 has 130 hp at 3800 rpm , the 25% smaller 200 has 121 hp at 4400 rpm. Technically, it should be only about 105 hp based on the 25% loss in capacity, but its only 9 hp off the 250. So destroking 25% with a longer rod is actually going to give an 8% loss in peak power. The bad news is that torque tumbles from 230 lb-ft at 2000 rpm to 177 lb-ft at 2400 rpm, a 29% loss, more than the capacity drop. If the car is geared to suit, the torque loss is irrelevent, as quarter mile times are dependaent of start line thrust and peak power...torque is never a factor when gearing is optimized.
If you run the engine analyser, the optimum engine size for ideal specific power is a 250 block with a 221 crank. Use a rod longer by 0.225 (6.105"), which would yield a 1.76:1 ratio, up from 1.50:1. There is no power loss for this optimized combination, but the rpm peak goes up about 200 to 400 rpm, and again, you loose torque. You gain in smoothness and rev range, and get a minor Specific power and brake specific fuel consumption improvement. One of the torsional vibration periods is moved up higher. On 250 Ford sixes, the rpm at which this happens is around 5300 rpm depending on combination. Reducing the stroke raises it up proportional to the stroke decrease.
Getting back to the US 200 crank into the US 250.
There are four things to do, all of them big jobs for a US 250 to US 200 crank conversion.
First, the US 200 crank is sleaved to suit the bigger US 250 timing gear and balancer,
Then the main bearing caps are fitted with either another set of scaped and linished US 200 F770 bearing as spacers, pined to the stock US 250 bearings with 1/8" steel roll pins or brass dowels
Optionally, the 200 crank needs its seven main bearings enlarged by a long run welds made in a downhand postion, and then remachined to stock 250 sizes.
Lastly, the 200 crank needs its flange long run welded up to the 3.622" daimeter from the stock 3.4". Its not a biggie.
Then you just use the big bell 164 tooth flexplate of flywheel, which has the smaller 200 cranks 2.75" bolt spacing. The 250 uses the 3" bolt centres sizes, so you just add a 1981 to 1984 3.3 plate, and your good to go.
I've done a 221 to 250 Aussie engine conversion. The cost was 1700 bucks back in 2001, which includes the pistons, rods and crank welding. I used forged Chevy 229/305 pistons, our 6.27" 3.3 Aussie rods, and main bearings welded up on the stock 221 crank. The crank flange where the flywheel meets still has to be welded up 0.220 to suit the block, and then machined back to the 250 size. A spacer won't work. I've tried making one up, and then looked at the other otions of a special crank seal, but they don't seal properly.