Be prepared and safe with an Interlock kit for your existing panel
If you have an FPE panel you could be out of luck. I looked high and low. You can change the guts to a newer brand. You will have trouble with your insurance comp. anyway. "they don't like FPE" not only are the breakers not accurate,  the panel dimensions are too small also. 
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Challenger Generator Interlock Kit  150 or 200 amp panels

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50 ft  6/3 - 50 AMP

Nema L14-30 Generator Power Cord 4 Wire

L14-30 Generator Power Cord 4 Wire 10 Gauge 125/250v 30 Amp 20 Feet

Miady 20ft 30 Amp Generator Cord, 10 Gau

30 Amp Generator Cord, 10 Gauge Heavy Duty L14-30

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PB30 30-Amp NEMA 3R Power Inlet Box for Generators Up to 7,500 Running Watts

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Adapts a 4-prong 30 AMP L14-30 receptacle to two -15 receptacles

Reliance Controls Corporation PB50 50-Am

Power Inlet Box, 50-Amp for Generators Up to 12,500 Running Watts