the technology has changed a lot in 35 years. while a deferent burner create more heat. The heat exchanger would not transfer it to the air. A good contractor should be able to disassemble the unit in place. Also you will more than likely find the new models to be smaller in size. Here is a link
Alpine Home Air Products: Contractor-grade heating, air conditioning and indoor-air quality products at wholesale prices. ... you can match out the output of your furnace. They have dimensions and you can see if it will fit. Remember 20% of your heating bill is not being used. If you lowered that by 15% how much would that save you next year .. the year after ... in 5 years ... i saw gas at $3.39 yesterday .. sorry i will get off my soap box