Building up an A10 case to take A65 barrels takes a huge amount of weld! tried it once years ago. Never finished it; the case distorted badly after about 2 hours welding!
An A10 barrel is best bored to a max of .060"........080" is possible. Went to 73mm (+.120") on one once, but the cylinders got a bit thin and nearly seized on one occasion due to localised hot spots.
As you say, an A50 head would be best; the 75mm bore A65 head would be too big for the bore.
If you do get an A50 head on there, remember that the pushrod /cam lobe order is different; the outer 2 are exhausts on the A65 but inlets on the A10. An A65 cam is not a straightforward fit.