Re: rear cats!
Regarding back pressure, you probably get enough from the bends in the manifold and exhaust system.
If you require extra back pressure with gutted/removed cats you can do the following:-
Install a 'thumb screw' (one of those with a football shaped head) on the inside of the exhaust.
This can then be turned to cross the stream, or be with the stream of exhaust gases.
By doing this you can fine tune the amount of back pressure.
If this doesn't create enough back pressure, then weld a regular flat washer onto a screw/bolt
and install it into the exhaust system as above.
If you don't understand these ramblings, let me know and I'll make a quick sketch and try and upload it.
I've done this on a motorcycle exhaust system, and it does work.