13 x 7 Rebels. Do they fit within stock Mk1 wheel arches?

Depends on how you was checking them, if the car was with dangling axle on one side it will poke out more, ya know angle n stuff.
If they was tested (tyres off) , the best way is bolt the rim on to the hub and sit the rim on 4in wood blocks and lower the car to resting height will give a more accurate assessment.
Rebel rims of 7in should be the same as say minilite, globes, crosswires etc.

Most of the Rebels I have seen tend to be fairly deep dished by default.
I think the rears would be fine, but 13x7 on front of a mk1 capri would most likely rub.
My 14x6 with 205/50  on minilights is about on the limit