If its an early Clifford bore-out on a pristine 250, and the machinist knew what he was doing, you could problably get away with it. The guys at Clifford have possibly forgotten more about US I-6'S than most of us would ever learn.
Any one else, and I'd say heck no, don't do it.
Any engine made in Windsor Ontario after 1962 is thinwall, 250 thou max jacket thickness. Just like every 221/260/289/302/351 and 240/300 and 200, it will have a 40 thou maximum, and 30 thou to be safe limit. I'd never do it.
You can find pristine Windsor castings which will go well over the recomended 40 thou maximum limit, but any thinwall engine has only about 120 to 180 thou of cylionder wall thickness when made, and less than 90 to 150 thou is common before a conservative 30 thou overbore. The casting around the machined cylinder is pitted sand, and over the years, the air in water forms a 20 to 30 thou scale which comes loose if treated with mutraic acid. This will hurt cylinder wall strength. According to most reputable engine builders (the late Austrlian engine designer Phil Irving, the late Brad Girdwood from Brads Speed Shop in Australia, and Sam Blumstien from Chevy Offroad & Marine Engines in Australia), an iron cylinder with 11:1 compression and about 1.5 hp per cube, will need 180 thou of wall thickness at the thrust surfaces. The only thinwall engine like that is a Chevy Small block. Fords engines are far worse than that. Nothing was done untill 1985, when Ford started having issues with there Windsor (Canadian), Lima (Mexican) and Cleveland (American) made engines. Since then, Ford has added more metal in every block, and looked at each block as a potential candate for remaching like Chevy has on every engine made since 1955. As a result, Chevy is brand A every time becasue of its cylinderwall thickness.
The upshot of all this? You could do over 250 hp with a 30 thou over 250 six with 120 thou wall thickness. I doubt you'd get any more than about 90 thou wall thickness with a 70 thou over 250, and its likely to be a risk at some stage unless someone has gone to a lot of trouble finding a very good casting.
The evidence from Does10's and some other 200 builders in the USA has said that 30 thou is perfectly safe, 100% of the time, but that 40 thou starts to get risky. The engine machinist can feel the degree of resistance when the engine is being rebored, and past 30 thou, the color of the iron casting often changes. The sand based cores can shift in thinwall blocks. It can be perfect or it can be very great, and cause even a 40 thou over engine to risk spitting No 1 cylinder under just normal driving conditions. Additionallly, if its from a frosty or snow laden state, it can be suffering from hairline cracks even if its stock and cared for for the last 30 years.
I've found that some Aussie 200 to 250 blocks can take huge overbores, because the blocks were made without much regard to cost on slow moving lines of less than 60 000 units a year, not 200 000 plus a year like US 250's. Even then, a 70 thou overbore would only be done if there was no other option, and you had a spare block. All Ford I6's and V8's don't have siamesed cylinders unless its a special SVO or Lima OHC block. The bore spacing is 4.08", the std bore 3.68, so the cylinders are very close together. There is no space for more than 200 thou of metal between each cylinder even if they were touching. They don't touch, they are sand cast apprat from each other in an eggshell section. So a 70 thou overbore on a perfect cylinder block like that would only have less than 165 thou maximum wall thickness on its date of birth, and less than about 135 thou 30 years on if its water wasn't changed regularly.
WARNING. Saving Iron for Environmental Reasons Rant follows:-
That's why I favour sleaving any Windsor I6 block for a common and cheap modern piston and rod assembly from a later model Ford V6 or I6 or Chevy V8. Any 250 engine, especially the US 260, is an investment, and I can't think of a better way to eliminate the problems with thinwall cylinders than by using a 2.9 V6 3.66" piston on reamed out 5.88" forged steel rods and Diesel Ford 2.3 Transit engine liners which you can make locally for very little money. They are something like 3.78" in outside diameter, 93 mm internal, and cope with 21:1 compression ratios.
The other option is using 4.8 "Generation 4" pistons by Zoliner. Mate this with the GM liner, and its 6.1" conrod.
There is also the prospect of using the latest 1.21" tall 3.7" daimeter 5.0 Modular SVO OHC piston, which is to be used in the comming 6.0 or 6.2 liter Modular V8 truck engines. You get the engine fully relinered to 3.88", and using a 90 thou thick liner should allow the 5.4 V8 conrods and the 5.0 modular piston to be.
These pistons have had millions spent on them by Ford and GM. Back in the late 90's, Ford and Chev had issues with warranties on cold piston slap in service. So they worked very hard with Zolliner to elimate it. Along with detonation reistant conrods and the brillant work on piston ring clearances, there isn't a better cast alloy piston around. It would be silly not to work out a price on doing an linered up 250 engine block, and eliminate the prospect of ever having to do an engine rebuild again. Linered-up engines are able to resist detonation better, and if the liner is thicker, they are stronger than a stock Ford cylinder wall.
Talk it over with your machinist.
