Dear M&M;
Your going to need to do some major research here.
First off; you won't need steel seats or special valves, unless your head is in really bad shape.
Next; For low RPM usage 'headers' won't be needed and I don't think Fenton made an exhaust manifold for the GMC. It they did, it will be a huge cost.
Please check with your Propane/LPG dealer first before you buy anything.
Welcome aboard & Good luck.
