When I bought my used Quadra lift model, the owner had disassembled it for shipping and for some reason had removed the lathe column casting rather than just the mill head assembly. When I reassembled the machine I aligned the headstock to the cross slide table and then put the pins back in place. I suggest you remove the pins ( mine were threaded so you could put a little bolt in them and use it to pull them out) and loosen the bolts a bit/ Then use a rubber mallet to move the headstock into alignment. Tighten the bolts a bit at a time and re-check after each sequence to be sure the head is not shifting. Once you have the bolts tight and the headstock is aligned, drop the pins back in- if they won't go in, DO NOT drive them in- this will move the headstock. If you have a selection of tapered reamers or can borrow one, just ream the hole and insert the pins. A tack weld at each corner is a quicky, but not so attractive way of doing the same thing. The bolts will probably hold the head in place without the pins as well.