We used the same pebble mat as Markebo1, then just added a stock rubber mat over it in the front trying to maintain a stock look. We also installed (from the top of the firewall to the top of the rear wall) butyl rubber w/aluminum sound mat (actually some roof repair material, then added closed cell foam insulation with aluminum backer insulation) now the truck is quiet and cool....cost about $2.50 per sq foot...........got it at home depot.....Kieth
http://kieth.smugmug.com/Trucks/66-GMC/i-SDQRkDS
http://kieth.smugmug.com/Trucks/66-GMC/i-HTG7Xwm
http://kieth.smugmug.com/Trucks/66-GMC/i-Zh5xGp3
http://kieth.smugmug.com/Trucks/66-GMC/i-XjWJ9tG
pebble mat
http://kieth.smugmug.com/Trucks/66-GMC/i-TJ6s3L4
pebble mat plus stock mat:
http://kieth.smugmug.com/Trucks/66-GMC/i-VBbz2pG
worked out great, quiet easy to clean and dry as a bone......Kieth