Just to start off I have to say that I'm not a trany tech and I don't claim to be one . I'm just talking from personal experance and the research that I've done on the subject . It go's like this . Starting in early 94 into 95 if this guy's truck has a 3.9 then it should have a 42RH in it and if it has a 5.2 then it should have eather a 42RH or a 46RH in it and if he has a 5.9 then it would have eather 46RH or a 47RH in it . Now I'm not saying you can't make your 518 work but starting in late 94 the trany is also controled by the computer and the people that I talked to that tried to do this swap ended up with problem's such as no over drive , shifting funky , lock up converter slipping , etc. etc . etc .. Now that said . If his mechanic is saying he can make your 518 work and he's willing to back it up then I say sell it to him . Because it sound's like you've done everything in your power to help this guy and he know's what he's buying . And if it does'nt work then it's on his mechanic not you . But what I would do is write out a "as is" bill of sale just to cover your end . I hope this helped . I know it can get very confuseing with this stuff . As far as the cost of rebuilding his trany ? My trany for my 95 cost me about $1700 with new converter and I did the R&R on it so that was just for the rebuild it's self .