I need a section off of a broken block, can I cut with an oxy-acetaline torch? I would cut a much larger section then grind it down to fit.
Can’t get to it with a grinder or saw

Years ago (a lot of years ago!) I tried cutting a Chevy three speed transmission housing with a gas torch. As I recall, it worked but wasn't pretty. Just throwing this out, have you considered a plasma torch?

My next choice but I’ll have to buy one

I would think a plasma cutter would make a cleaner cut.

Would a sawzall work ?? Pic's or a description of the area you're after would help

While you can cut cast iron with a plasma cutter torch, or any torch.it is not the best method to cut cast iron, due to cast poor Iron content.
When cutting cast iron, it is best to use a snap cutter, angle grinder, or reciprocating saw.

Cutting out a part of the cylinder in the bottom of the cylinder inside the crankcase to weld back into another block

Maybe something like this?

Angle Cutter

I'd use a .030 or .060" cut off wheel on a 4 1/2" grinder.

I thought of one of those chainsaw type cutters they use in the wrecking yards and rescue trucks too. The piece is right up into the cam area

What kind of block are you trying to fix?

If you can find someone who is capable of making this repair then he will know how to cut out the part you need. Note the word if at the beginning of my post. That is going to be a very difficult repair in cast iron. Is this block super rare and unobtainable?

Hemi and rare. My fire pit is made to get to the proper temp, I have the proper rods and the skill, I will fix this. The ‘piece’ will be from a broken Hemi block so the same composition. I’ll try the sawsall first, just a lot has to be removed first

I'd avoid cutting grey cast iron with a torch. From my metallurgy classes back in college, I seem to remember something about that converting grey cast iron into brittle white iron. That is why special rods are needed for welding cast iron.

Air arc.

I would try to do the job with a 4 inch O.D. abrasive cut off wheels, as many as needed
Take your time and bring a lot of patience

Cut it with a reciprocating saw if you intend to weld the cut piece into something else. Cutting with either a torch or a plasma cutter is going to give you a cut edge that probably won't weld well.

How do you intend to seal the water jacket of the motor once you cut and weld it back together? If its the bottom of a cylinder your trying to repair, wouldn't a sleeve work much better, or is that big of a chunk broke out? Gene

Getting to hot on the cut worried me too, reason I asked. To answer a few questions there’s a good size hole in the bottom of the cylinder (that someone tried to fix before I got it, a wire welder no less) and it will get a sleeve after. Block will be brought up to around 600-800 degrees, welded, then covered with the coals for a long cool down (the secret for no cracks). Then pressure checked for leaks and magged for cracks. Actually got two welds in that cylinder, a vertical crack from bottom to within an inch of the deck opposite the hole.
Block has all the right numbers

Boy, that sounds like a block I ran into at one time. Crack in bore, attempted repair with a MIG, other damage. Here are some photos I took before the seller got the block back.