header/link pipe question

[Pete]

Howdy!

Gonna be undertaking my underseat conversion plan (twin underseat blueflames) in the next couple of weeks and I've a bit of a question:

Is there any rule I have to follow when getting link pipes made so's not to reduce bakpressure, performance or anything???

Also, can anyone recommend a good cheap method of shielding my undertray from the heat of the cans?

Help???