I just installed custom http://www.speedhut.com/ gauges in my e-body. GPS speedo is great, change gear ratio, tire sizes and speedo is not effected. They will work with your custom idea's /designs. Great company IMO.
I just welded up an oil pan for a guy with a 454 Chevy that when he was changing the cam dropped a lifter and never found it, replaced the lifter, fired the engine and it spit out the side of his new oil pan like a bullet and destroyed the starter. The hole looked like it was perfectly punched...