I just dug out both Thermoquads I have.

1 is a 6518s which appears to be a 440 industrial carb if my data is correct.

The 2nd is a 9342s and appears to be different vent/emissions.
I haven't been able to determine what it was off of. I assume an early/mid 80's truck

My plans are to mount a pair on a tunnel ram.

Is the 6518 better and I should seek another one to match?

They are both described here...different primary sizes looks like...don't know if this helps. If not it bumps it up at least.

web page

Thanks! I see I wasn't far off on my guess on the 2nd one. later emission carb with bowl vent.
I'll have to keep my eyes open for another 6518 or something close with 1 1/2" secondaries. Well that or another Edelbrock 600cfm since I have one of them as well.

6518 H-440-3 Chrysler Industrial 1-1/2 1

9342 82 318 Truck BV (4287013) 1-3/8 3