**"dilkey"-* you say its a 1978 model? dont invest to much more money on a old inefficient unit, 400dollars to replace the burner tubes, now a spark box issue, then whats next, do you see where im going? these are only my thougts, im here to help people, good luck and keep us up to date.*/-*
I would agree why spend money on 1978 furnace with most likely thin/wall heat exchanger and no model/serial numbers.
Upgrade system to new Rheem or maybe a Carrier variable drv and you could save enough to pay for furnace in maybe 3-4 years.