He has subsequently added that he could insert a bolt whilst filling and that the aluminium wouldn't stick to it, so it would be effectively tapped if that would work?For the 50 quid is just welded or welded and taped ? If the whole job sounds good to me after all it's an engine mount you would not want to to brake ?
Would you have any concern over the enlarged bolt hole not supporting and restraining the bolt?if there's clearance there..... I'd put bolts through from the inside... it'll take 2 min, doesen't need any tools you don't have and you know it'll work right away, no worrying about stripping threads etc.