Not necessarily instruction without a pic, just some things to think about if you haven't BTDT as many of us have:
If you can get the pump off ok leave as much of each stud there to grab onto after heating or for welding on a nut. Extractors are the next step if you get down to flush. I've had better luck with square-ish straight fluted ones than the spiral Easy-Outs. They seem to grip better vs carving a tapered hole, esp if they cant off-center when reefed on. Chances are such studs/bolts will be 8.9 grade and can be dealt with as one would a US Grade 5.
After soaking in Kroil, Gibbs, etc and/or heating be sure to rap the stud a few times as if driving a nail to shake things up. When doing that, or if you're heating a protruding remainder, grind minimal flats or at least get it to a bit of an oval shape before you vice-grip it or the jaws may just whittle the nub smaller. Twist gently in & out if possible to loosen things up, too. When the guys say 'lotsa' heat you'd do well to let surrounding metal warm up quite a bit, too ... er short of melting anything like dripping wax. If under-done you'd only heat the protruding end and not the thread deeper inside where it does the most good.
If you end up drilling 'cuz there's nothing sticking out, go fairly big whatever you plan to do from there. I'd use a 4mm drill for a 6mm bolt, 6mm drill for an 8mm bolt, etc. Square the end up and center punch to get going in good position & start with a 'stub-length' or center drill vs std or 'jobber' length. Use pressure to slow the drill down & wet it with ATF or similar. Watch you chips to see that you are cutting ok.
Put up that pic for us to get the best help. IMO you dare not think of this as an easy fix that might be difficult, but rather as a tough one that might go easily. Hopefully the radiator, etc won't be in the way. If so, I'd be willing to pull it to really get at the problem....
