Yeah, unfortunately you are right, unless the motor was bone stock, a 3/71 is too small for a V8, even a 302.
They simply don't flow enough air to feed 5 Litres.
As they are not efficient in design either, you couldn't overdrive one fast enough to get the required airflow you need.
For any V8 of 5 litres or more, a 6/71 is required. Tough 351's will use 8/71's or even larger for drag use.
Keep an eye on ebay or the trading post. Be careful with used ones as many need a major overhaul, new teflon seals etc, bearing kits or machining if foreign bodies have been sucked in to them.
Maybe talk to some reputable speed shops, they often have used ones lying around and may be able to help you out.