Do stock unported 1968-71 340 heads flow about the same as the Big block 906 Heads?
Thank you.

Here is a list of reported flow numbers, but it's hard to really make a comparison as I can't tell if the big block and small block numbers were flowed with the same machine, procedure, and port radius or not?

http://users.erols.com/srweiss/tablehdc.htm

If your comparing stock to stock, then the numbers to look at would be around 0.400" to 0.500" because the stock cams only opened to mid .400 lifts
340 is 0.429"/0.444" lift, 268/276 duration - J head at 0.400" looks to be about 190 cfm, 0.500" maybe 200 cfm?
440 is 0.450"/0.455" lift 268/284 duration - small valve 516 looks to be about 10 cfm better? maybe 200/210 cfm?
The 906/452 looks to be about 25+ cfm better, around 215/225 cfm?

Depends on what you mean by “about the same”.

The best stock 906 I’ve tested was 20-25cfm better than the best stock X head I’ve tested.

Without looking through my notes, I’d say around 10-15cfm difference would be typical between a 906 and an X.

Through the stock mani, they flow very similar which is why the 68-70 340 has almost the same NHRA HP factor as the 68-69 383.

Isnt the factory weight of the car the motor comes in a consideration as well when factoring?

Factory weight and HP rating are used to find the car's natural class, NHRA may change that factor higher or lower depending on how well the average car of that combo runs, that's the very simplified explanation. As an example the 68 Barracuda with a 340 was factored to 299 from 275, with the 383 it went down to 295 from 300. so the 68 Barracuda with a 340 would have to weigh 42 lbs more in F/SA @ 10.5lbs per HP which is where they frequently run. Also note that the same cars in Super Stock would have the 383 @ 19 HP more than the 340 where the stock mani is no longer the equalizer.

You can't go off NHRA's numbers as factual. My 72 low compression 340 has a factory rating of 240 but NHRA rates it at 293. Lots of politics involved on HP factors!

There are politics for sure but in spite of the racers trying to protect themselves from the AHFS these cars have been running for 50 years and are pretty well sorted out at this point.The NHRA factors are pretty good and if anything they are underfactored. The Thermoquad and mani are both excellent pieces compared to the AVS and it's mani. A 64 Ply 383 and street wedge are almost identical in rating with the 383 at 292 and the 426 at 291, same carb, mani type, cam and heads and they run almost the same.

If I read the question right, no, the car weight does not factor into the engine HP factor. Two separate factors.
A long time 340 stock racer convinced me to go with the 71 TQ instead of the 70 AVS, why? He said the AVS pans out at 6500 rpm, the TQ will go past 8500. Just one of many ‘tips’ he gladly gave me to rethink my whole program. After 50 years of racing the 340 he should know. IMO The TQ stock intake is the best flowing (for mass production).