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Number Dummy usually provides this service, but he must have missed you. I'll get you close and he or someone else will come along with more detail. First of all, you left one number out of your VIN which affects your build month. It will be one of two months which I will get to in a moment. F10 = F100 two wheel drive truck. B = 300 cubic inch 1V I-6 engine which you say is no longer correct. R = built at San Jose, California assembly plant. 68_032 = April or May 1965 build date. 129 = wheelbase in inches. T = Sandshell Beige exterior color. F100 = 5000 pound GVW rated half ton truck. M81 = beige upholstery. F = 4 speed manual transmission. A9 = Dana 60-2 limited slip differential with 3.54 gears. 5000 lbs = gross vehicle weight rating. 150@ 36,000 rpms!!! is twice as many rpms as a Formula One engine turns, but does not apply to your present engine. The 352 - 2V engine installed in '65 F100s was rated at 172 NET horsepower (208 gross) at 4000 rpms. 74 = Seattle, the district sales office which ordered your truck when new. It was not a special order vehicle. Hope this helps.
You can find the VIN stamped on the TOP of the passenger side frame rail, approximately 40 inches back from the front end of the rail (where the front bumper attaches). There is usually a second one further back on the same rail, but it's under the cab and hard/impossible to see without removing the cab. Be careful when you're removing layers of crud from the top of the rail that you don't damage the stamping. The VIN will be preceded by and followed by stamped stars (*). I'll attach a picture, but it will be hard to see. Hope this helps.