I hate to say it, but there is no such block prebuilt. To have them made would probably cost thousands of dollars each (a good 20" block is SEVERAL hundred dollars, and they are set up to make them in quantity. Mind you that it can be done for a price, but the lead time will be long.
Another problem is that special sizes usually require a minimum of 4 be built (something to do with the lapping process was what I was told).
My suggestion would be to PERMANENTLY wring blocks together. For example:
24" - (2) 12" blocks
40" - (2) 20" blocks
Both sets should (must) be wrung and held with rods (use square blocks), using threadlocking compound on the threads to 'preclude adjustment'.
Most good dimensional labs should be able to measure the finished stack for you with an uncertainty of <100µin (at 20°C - your temperature and uncertainty may vary, offer void in some areas). The manufacturer may even be able to do this for you, and it would be much cheaper than being made of a continuous piece of metal.
More for the 'information' file:
Clean, well wrung gage blocks begin to go through a process of 'cohesion' in a fairly short time. Cohesion is the process of two separate masses sharing atoms. I've seen it in practice (purely accidentally - we couldn't get blocks apart with a rather large mallet - they had been wrung together for only two days).
Ryan
(Since you're in Indiana, and I lived/worked there in a purely dimensional lab until April, I can put you in contact with someone that can help you explore this further, and it is an accredited lab. I will not post the company in the forum, however.)
