my I beams are 4 ft. I have a 9" dia. gen pulley with 23 1/2" dia. fly wheels. The belt # is 5060970 which = a 97 " belt. the gen mount floats on the I beams. the beams are tied together with 3"x5"box steel about 20"long each. the gen frame is made of 1/2"x2" bar stock to ride on the I beam flats with bar stock below to parallel the inner I beam to form a box mount for generator. I made a bracket attached to gen mount which has 5/8" all thread running through the box steel with nuts with a heavy spring for belt tension. there is 2 bolt with flat stock tabs to lock the gen frame in place once tension is good. the block is bolted to I beam, the gen mount floats on I beam. (these measurements: engine and gen. mounts can vary! the cross member measurement is determined by the engine block mount holes! this is a serpintine belt common to Gm cars the pulley is a 6 grove.
The info is on the CD with pics, finding it is an issue I found it a couple of times, once with a google search, and once on one of my 2 George CDs.
Have fun! hope this helps!