v6-v8 wiring the same ??

Basically, yes, they should all be the same. Only differences is in the engine bay area, the location of the connectors where the sending units and component locations are.(i.e. coil, Ign box, etc..) On a customer's car that was a V6 swapped to a V8, I just unraveled the harness and rerouted the wiring and shortened/lengthened where it was needed with soldered connections and heat shrink. Then taped it all back up to look just like factory. Only thing of concern would be the sending unit ends. I do believe the plugs are the same even though the sending unit bodies are different corresponding to the engine they're fitted for, just can't remember 100% for sure.

Hope this helps.

So could I hook up a new off-the-shelf Painless wiring harness up to a '71 302? Or would I have to get one custom made?

Can't say for sure. I know Painless does make wiring kits for specific car models, just not sure of your 71. (or are you talking just the 71 engine in your II??- In that case, what's different about the '71 302 from the one you have? -Or is this a V6-8 swap?) <--- More info please..
On the norm I work with the universal kits because most of the time the wiring being replaced is in something either 6-volt or just basic or oddball like older Jeeps and Toyotas. I've used univeral kits in a couple of SBC powered 60's Toyota Landcrusiers. Nice kits with lots of extra length and plenty of circuits. I have a Ron Francis universal harness in my '25 C-Cab and it's a pretty nice setup aswell. However, if you want a stock look and stock installation, with stock hookups, this may not be the way you'd want to go. I do believe there's gotta be an aftermarket company re-poping replacement original harnesses ............somewhere!!!

However in most 60's-70's cars, especially Fords, i make a hard attempt to repair what's screwed up since Ford's electrical system is very easy and extremely simple. In my opinion, anyway.
Heck, you want a challange??? Try repairing a butchered 60-70's Mopar harness... it's a nightmare all in it's own... but I did it.

Well, I was asking because I have a '78 with the '71 motor, and the current wiring looks like it was installed by an Amish electrician.

Believe it or not, there's really no difference between the wiring (basically) from 1962-74 SBFs other than the connections at the sending units, their locations and the later coil style. They're pretty much wired up all the same. And from '74 to '78, just add in some of the smog crap aswell as an A/C realated device or two added to the wiring.

If the amish have invaded your engine bay it could have been because your car was originally an I-4 or a V6, or some wiseguy, Capt. Buttconnector thought he'd add some nice JC Witney garbage to the engine bay or who knows.... some of the stuff I've seen over the years... unbelieveable...

I thought it would be the same...yes, it was originally a four-cylinder. It's all original besides the 302.

My girlfriend Jen says that she's going to start calling me Capt. Buttconnector now.