I just got done taking mine off. I would weld the y-pipe to the crossover. Adjust it and mark it to weld together so that when the y-pipe is hooked into the rear header that it lifts the crossover up and away from the top of the tranny as it is very tight in there. Use a band clamp connecting it to the rear header and make sure that you orient the bolts on the clamp so they are away from the firewall when you tighten them. You could do it without RTV, but it wouldn't hurt anything to put some in there when you clamp it in.
You should be able to just feed it in from the top and just hook it. You also don't have to take the front or rear header off to get the crossover piece out when the crossover is welded to the y.
The pacesetters would be a ***** to get in/out if you welded the crossover and the rear header to the y pipe.
