I did a little autometer gauge project in my spare time. I used 3 3/8 speedo and tach and 2 1/16 fuel, oil, water. The small gauges were very straightforward and nothing custom was needed. For the speedo and tach I made a support out of 3/32 ABS plastic that I bought off ebay. Obviously a 5 inch tach/speedo is ideal but I got a smoking deal on the 3 3/8 gauges. And for a 250$ total investment I couldn't be any happier.