Not to question you Kelley, but I've never noticed a difference on the exhaust ports between the 7120 or 0630, nor a difference with the headers between the two(I've spent most my time w/ 91-96s). Now when talking of using a 7120 or 0630 in place of the 0331 w/ small exhaust ports it is true that the smaller 0331 exhaust manifold will not cover the larger exhaust ports of the 7120 or 0630. To make matters more interesting, my 1/96 0630 head has the coolant port in the back of the head and sensor to feed dash gauge. The 97s with their newer style dash and bus system did away with that rear coolant sensor in the 0630 and got the info from the cts in the t-stat housing.
Hm - OK. I haven't gotten a 7120 head in stock yet for direct comparison. It may be that the
volume of the port was reduced without reducing the port
exit significantly, and for a similar reason to why they reduced the port further for the 0331 head (maintain exhaust gas temperature retention to improve catalyst light-off time.)
I recall noting somewhere that the 0630 port was smaller than the 7120 - but that could be it (or I could just have my wires crossed. That's why I'm collecting heads. Anyone near San Jose, CA with a cracked/warped 7120 and/or 0331? I have a 2685, two 2686, and an 0630 already...)
It's worth noting that the 0630 head still has the cast boss for the gage driver, it just isn't drilled/tapped. I have yet to check this, but it's fairly likely that the head would support having that spot drilled/tapped 1/8" NPT as well, if you don't mind doing the work (just be careful how deep you run the tap. The taper can cause you trouble if you go too far down...)
I find it interesting that you've got an "early 0630" head that still seems to be OBD-I - as you noted, it was OBD-II that used the unified CAN bus to drive gages and engine management. Which style of IP do you have? The fact that you've got that hole drilled/tapped, tho, lends weight to the previous argument (the boss is simply unmachined, and that may be corrected at will.)
