http://www.electronicsandbooks.com/eab3/manual/Magazine/Q/QST%20US/195602%20[156].pdf doesn't work as a hotlink because of the brackets around [156] in the url.
However, it is a link to a 1956 issue of QST magazine, and there is both a nice 6146 transmitter article and an article explaining and detailing the modifications to an MB-40-SL grid circuit, including mention of thermal damage occurring at 70W before the modification.
I'm still thinking a fun little 2x2E26 plate mod with the -SL as the output would be cool. 2 would do about 40W carrier, , and two more in the modulator could make 40W of audio easily.
40W of AM might seem silly, but it's enough to play with-and it'll take me a few years more to finish this anyway. Just something to occupy the brain.