I'm with the others when it comes to the level of racket you can tolerate out on the highway - also when it comes to wear and tear on the engine.
Another factor to consider when ordering your Tremec is at what RPM does your cam start to make power. If you order the tranny with the .68 5th gear - will the engine be loping and stumbling in 5th gear at highway speeds given the rear end ratio you have?
Here is a little bit of little known trivia about Top Loaders and Tremec 3550's and their variants;
When the Top Loader was first being developed back in the early 60's, there was a young engineer by the name of Julio who was on the project team. Today, Tremec transmissions are built by Dana Spicer / Tremec and the Chief engineer who brought the TR3550 to the table was none other than Julio!.
I had a considerable amount of conversation with Tremec over this transmission as I was an advisor to the SCCA's American Sedan committee and we were considering the Tremec for a legal "Alternate" transmission to the weak T5 (Also manufactured by DSA's Tremec division). When I last spoke with Julio about the TR3550, he told me about this and went on to tell me that the TR3550 is, essentially, the Top-Loader 4 speed transmission with a 5th gear hung off the output / main shaft.
So - In a sense - If you hang a Tremec off the back of your bellhousing, you are still using the Top Loader 4 speed!
