Here is one article about the 261 from Jim Carter's site. . There is also lots of great information in "Tim's Trucks" great web site. It has been a great help to me, and I have running or owning 261s since 1958. Here is another classic 261 article.

A 261 will fairly easily replace a 216. If the 261 is a 2nd series 55 or later you may need a water pump adapter.

I'm not sure if a foot starter could be easily used with a 700R4. perhaps someone else can help here.

Hoyt