I am replacing one of the heads (in addition to the block) that has metal eaten away between two cylinders. Since I am re-using the pushrods and lifters, should I put the old rocker arm that is matched with the push rods on the new head? Im not sure if it would be a greater problem that it is not matched with the valve ends. Could the old rocker arm have been damaged from overheating?

 

Generally, Rocker arms are ok to reuse. It has been a while but you need to check for play or obvious damage, the biggest issue is clogged oil passages and damage to the cups the push rods sit in
You can use a nice clear oil and move the arms to one side to check for excessive wear as well.

If you have a shop you can flush them with solvents, then a very lightweight oil to flush the solvent. If not you can soak them in a good solvent, for at least a day move them around in the solvent, pick them up let the solvent drain etc then soak some more. Then soak them in very lightweight motor oil, cheap is fine, the same idea move them around lift them out let them drain put them back in, an hour or so is good you just want the oil to displace any remaining solvent

The biggest issue is finding a good set if you choose to replace then, there are some very bad cheap sets around that will fail in short order

 

Rocker arms can wear into the rocker shaft (on the pressure side), so check the shaft for wear and replace if needed (more than 2 mil or so). RPI Engineering has a good write up about Rover V-8 Engine wear. They recommend replacing rocker arms if the shaft has worn out due to bits of steel that get embedded into the rocker arm bearing surfaces, so just replacing the shafts doesn't help much.

Are you planning to rebuild this engine again down the road or sell it quickly? Camshaft and lifters wear out, so think about replacing those too. Lifters especially are supposed to be flat.

 

I think I may have been a little unclear. The new head comes with its rocker arm. I am wondering if I should use that one, or swap it with the rocker arm from the old head that it is replacing. The rocker arm from the old head is matched to the pushrods.

 

I think we have some misunderstanding about terminology.

A rocker arm actuates a single valve.

On a pushrod V8, a rocker shaft has eight rocker arms mounted on it.

 

Personally, I would use the new rocker arms. Others might have different opinions.

The pushrods will stick into the rocker and lifter cups with oil present to create a suction. If they don't stick, that's a problem, but if they stick, then you're good to go.

The Rover V-8 is not a race car engine, so mixing pushrods, lifters, and rockers is OK IMO, even though I try to keep everything matched up. If a part is replaced, it doesn't "match" anymore, so don't worry about that.