Trunk hardware

Well that's a very unfair conclusion (mickey mouse) about tires based on its dimensional size. But people do that all that time. yes 30 years is a long time for a tire but its condition depends on environmental factors and their impact on the materials.
1975 Operators manual shows the same tire sizes were still being offered, as well as optional E load range tires.
The problem when converting to p-metric radials is the construction results in a different dimensions.
For example P185R14 has a section width of 7.2" and static diameter under 25". Load rating is fine for normal use.
P195/75r14 is a common replacement - probably the one they will suggest at the typical tire center if you pulled in with an D78 and told 'em you wanted a set of new tires. That's 7.7" section width and 25.5" diameter (summer tire). That's about 1/2" taller in the trunk than the bias ply but closer in diameter. (Probably closer in rolling radius too but that's another ball of wax way off topic here).