Some pics here:
http://www.tomtownsend-toyland.com/toyland/m-5_tractor.htm
http://www.jedsite.info/tractors/mike-number-us/m5_series/m5-series.html
http://www.jagdtiger.de/index2.htm?http://www.jagdtiger.de/USTanks/M5HST-01.htm
http://www.kmk-scaleworld.be/walkAround/M5-HST/
http://www.jedsite.info/tractors/mike-number-us/m5_series/m5-hst/m5hst-intro.html
http://www.jedsite.info/tractors/mike-number-us/m5_series/m5-hst-ringmount/m5hstrm-intro.html
Lend Lease version here
http://www.o5m6.de/m5_hst.html
From what I have read, they didn't pull the Long Tom gun but the 2 wheeled 155mm howitzer. Not sure if this is true but only pics I have seen have the smaller gun. Supposedly the M4 tractor was the prime mover for the Long Tom.