I would suggest 03, 08, 22, 25, 35, 42, 43, 45, 46, 47, 52.
1971 is too early for the 37s, but not for the 25s. However, the diesel-electric classes 45, 46 and 47 had started to appear by then
Hymeks (35s) were rare, but they did occasionally run into Cornwall, usually deputising for a failed member of another class.
St Blazey had 08s D4007, D4008 and D4009 (among others) so one of those would be good.
The 22s weren't very reliable. One of their nicknames was Bunsens because they often caught fire!
The furthest west I saw a 14 was Taunton and that was only once. Besides, if your period starts in 1969 they had all gone froom the main line by then.
Hope this helps.
Geoff Endacott