If you are running a 3.07 behind a 235 you are not going to have enough hp to do any good with an overdrive. Back in their day 235's were run all day long at 60 mph with 4.11 rears and it didn't hurt anything. You just have to get used to the engine noise. You say when it comes to replacement time. SM420's are so tough that you will probably never wear it out. My only objection to them is that reverse is so slow, but I live with that. My school car in the late 50's was a 50 GMC with a SM420 and I abused it. Everything broke but the transmission. I still have several GMC's with SM420's. I don't even dream about doing anything to them. I am building a GMC with a 302 that is getting a Borg Warner 3 speed overdrive behind it, but its getting 3.90. I am very partial to the BW overdrives. I ran many of them behind Y-block Fords and I still have 3 Y-blocks with od and 3.89. As far as speed goes, when I was a teenager I used to peg out my GMC at 80. My parents always told me not to drive faster than my Guardian Angel could fly. I must have had a pretty fast Guardian Angel.
