I have an older Sears Ten furnace, Model #4276451, Manufacture #96709, Serial #H82729917. I need to know how many input BTU's this furnace has.

Input btus should be located on same tag as model and serial info. Most likely this tag is inside the furnace on the side or bottom. It may also give BTU output.(which is really the important number to look at)

I presume you mean BTU output, not input. Actual BTU output may vary depending if you are using NG or propane. Why are you interested?