You could try casting the lugs onto your existing rockers if it's a bit shitty to weld. You'd have to clean it up well and heat it up so not to crack. We used to do that with even cast iron on occasion, You have a bigger joint / contact area compared to a welded piece too.I had thought about that, I've actually got a foundry and could cast them myself, but I couldn't figure out how to tap the tappet cover holes, I mean, I could figure it out, but it would involve a shit ton of money to make or buy a large tap, so it was that or redesign the tappet covers to use machine screws, I'll try welding for now and if it's not the right way to go, consider other options
Bed the rocker into moulding sand and make a pool around where you need the lugs. Get some heat into the mould with a propane torch and it should work ok. Any contaminants in your existing casting will vent to the surface, so you would require tabs far longer than you actually need, so that when you cut them to length, you're into clean metal. May be worth running a small riser at an angle from the bottom to help vent and get some flow across it.
If you bed it well enough on the underside, you won't have problem with burning through.