...how many more miles should my engine last? The car is a 1984 GSL-SE with 83K miles.

 

189k, 84SE - you should have some life left in that engine.

 

if the oils for sure are goin you have between 1-2000 miles left on it. It seems to depend on how you drive. i sronngly recomend against compression breaking. My seals were going in one 12a I had and I was going about 70 put it in 4th and was coasting down and the whole seal went. bang no power and you couldn't see 2 ft behind me. the biggest cloud of smoke ever. It still ran too, but you'd burn a quart of oil in about 5-10 minutes. I was Bondish with my somke screen.