My father in law ran two GMCs until 1968 as lime spreaders. They hauled chalk from quarry to farm and then spread on fields so they always carried a lot more weight than they were designed for. One had a Perkins engine which was a Perkins conversion kit. The other was fitted with a Ford engine and to do this the transfer box was moved back. Transfer boxes were hardly ever any trouble. As Tony says they were indeed ran on Chevrolet diffs again not a lot of trouble apart from a few bearing troubles. They always ran on 900 x 20 tyres, rear trunnions were troublesome. Ransomes of Ipswich made some stronger ones which seem to cure the problem. Probably the biggest problem was the short intermediate shaft between the gear box and transfer box mainly due we think to using English UJs. The grease nipple holes were bigger than American ones and this is where they seem to break. We found we normally broke them when spreading on freshly ploughed land where they were sinking in and getting plenty of traction. When considering the work that they were doing they coped very well. I can see no problem with the above conversion as someone who has earnt a living in the past using one, it looks an excellent job.