rear speaker questions

[DGW]

I have a 77 911 with an aftermarket stereo. It has an ADS P840.2 amp under the passenger seat and it had MB Quart discus DCE113 speakers in the rear. Information on the speakers indicate Impedance of 4 ohms and power of 35 to 90 watts.

The rear shelf was seriously hacked and trashed by some PO for these speakers. I found an original rear deck to put in the car. I would like to put stock sized rear speakers that fit under the rear deck but something that the amp wont blow up. Is this even possible and if so what parameters must the speakers meet to survive the amp. From reading some threads here is sounds like stock speaker size is 4x6 but also 5x7 should fit. Is this true? Also, do I need to buy shallow mount speakers? The sound pad under the rear deck has had holes hacked in it for the Quart speakers.

I hardly ever use this radio but would like to have it work if I ever want to listen to it. I usually just listen to the rarlyL8 exhaust.