I'm trying to make my 1st gen Firebird a little more highway friendly. My '67 has the Saginaw 4 speed with 3.55 rear gears and I'm running the tallest tires I can squeeze under it (275/60 x 15) but the engine is still turning almost 3500 rpms at 60 mph. I know I have options for making this car easier to deal with on long trips without killing my performance but they're all so damned expensive! Here's the options I've come up with and their pros and cons.

Tremec 5 or 6 speed conversion. This seems like THE best option for lowering my highway rpms (and keeping my tire smoking abilities) and for a mere $3600 (which, by the way, is more than I have in the ENTIRE car) I could make this happen. My budget says this is the worst idea imaginable. I could cut the cost if I could find a TKO 500 or 600 used but so far I haven't had any luck.

Gear Vendor overdrive. This is the one I'd go with if I had a Muncie trans instead of the Saginaw and virtually limitless funds. I could keep all the grunt, and with the press of a button, lower my highway cruising rpms considerably. Unfortunately, it's almost as expensive as the 5 speed conversion.

Doug Nash 5 speed swap. Still pretty damned expensive and I'd have to change my 3.55s for something closer to 2.76s which, correct me if I'm wrong, requires a different carrier than what I currently have. This plan would really only work if I could find an old trans that I could pick up much cheaper than a new one.

Last, and cheapest, is just swapping out my rear gears but that compromises everything.

Does anyone out there have a better option or am I just too damned broke to have the best of both worlds?

Thanks for taking the time to read this and (hopefully) giving me a better solution.