Re: xy 351c extractors???
For starters, Pacemaker 4into1s will fit an XY - model 4075, I think. Gammaboy has a set in his XY GT Rep, anyway. Would be worth sending him a PM and asking for a couple of shots of his set up - I'm sure he'd oblige.
As for power gains, the rule of thumb is about a 10% power gain, but there are so many variables to take into account that it's damn near impossible giving an accurate figure. Fact is, you WILL make more power and your car WILL sound better. That ought to be good enough.
Finding them secondhand... good luck! I scored my Pacemaker TriYs for $120, but I'm a very lucky boy indeed. And if you can find a set of TriYs to fit instead of 4into1s, I'd get them - more streetable, apparently.
Oh, and finally, find a copy of Performance Ford, Volume 4 Edition 7. In it, they show a photoset of fitting Pacemaker TriYs to... yep, an XY GT Rep.