Now that I have the thorleys I am thinking of going all the way back with stainless. This brings up a few thoughts.
1) I was thinking of 3" all the way but I thought I read in another thread that 2.5 was better.
2) This came with no catalytic converters. I thought I could add 3" universal freeflow (Summit) then 3" freeflow mufflers and then all the way in the back cris crossing behind the rear axle one of those Duramax freeflow X mufflers.
3) The thought is a lot of freeflow items X pipe all the way back, lots of torque, cooler running and cleaner air.
4) Any problems with any of this?
Thanks, Lloyd. 1987 33' P32ish 454 carb w/tag

...all the way in the back cris crossing behind the rear axle one of those Duramax freeflow X mufflers.

My understanding is: to get the "crossover" (X-pipe) benefits the crossover, whether "H" or "X", should be up front, as close to the engine as possible, and definitely ahead of the mufflers. Even "free-flow" mufflers will change -slow- the exhaust, and the benefit of the crossover is the suction it provides.
Thorley headers already scavenge the exhaust; that is one of their several selling points (I had a set way back, on a '68 AMX, and the difference was dramatic).

This is what I thought until I asked some questions about X pipes in another thread. Then it came up that the further back the greater the benefit?? Which I hope is true.
(If you are fabricating your own X-pipe, a tip is to place the X-pipe union as far to the rear of the car as physically possible—it will make more power that way.)
Since then I have read with H pipes the closer to the motor is most benifical.
That bring up another question. If the X pipe works better more towards the rear, does it have to be before the mufflers and/or catalytic converters?
I guess more research is in order. Thanks Lloyd

I just noticed I did not make myself clear on the part of adding catalytic converters? I am not sure if this vehicle had them originally or not. Is there any problem adding a set of large free flow converters to this vehicle?