Do a search for Jarvis. They make a bull barrel for the $597, it runs $198 in either blued or stainless.
You will have to contact barrel makers or visit their web sites to see if anyone is making them or would be willing to. Jarvis makes aftermarket "Bull" barrels for the Remington 597 in .22lr, but they no longer make the .22 Magnum barrel - I bought the last one a year ago
It will vary with the maker of the bull barrel.
You will probably have to contact a barrel maker.
Barrel diameter
ADL OR BDL? There is a difference.
A while back there was a long action version of the Savage Model 12 (I think it was the 112BVSS) that was chambered in 7mm Remington Magnum. The last I ever really heard of them was back in the late 1990s. I've not seen anything about it since then. For now the only manufacturer that makes a bull barrel 7mm Remington Magnum is Remington, their Sendero line, as a matter of fact.
Just picked up a savage .270 WIN. in a model 116 with bull barrel. From Shiptons Big R in Billings,Montana.PH:406-652-9118.
100-500
Sure. A Sportsman 78 is just an ugly 700 with a cheaper trigger. Any 700 part can be used on it.
You will need a slide/ barrel bushing that will fit the barrel you want to install.
A heavier (bull) barrel is stiffer than a light barrel. It does not vibrate during firing as a light barrel will. Combined with a steadier aim due to the increased weight, the combination produces greater accuracy.
bbl