Hi Paul, I have limited experience with the '67 motor, but some. Take this for what it's worth.
I believe the difference in the studs is the length of the lower thread making the center unthreaded part longer. Studs are lightly bottomed in the threads during install. I use Loctite 243 (eBay
) or 242 (easy to get) to keep oil from wicking up the open threads in the center 4 studs & leaking out the top.
The 2) studs at dowels # E3523 (70-3523) have the shorter thread at bottom & wider center.
The 4) other studs are # E672 (70-0672).
As was stated the 2 dowel sleeves locate # E1534A locate cyl in relation to crank shaft so bores stay parallel to crank. That's important in my mind. So we want to save the diameter of the hole for the hollow dowels.
As you can see the lower threads are coarse, upper are fine. It is my understanding these are British Standard Whitworth 3/8-16 with 55deg thread angle.
There is a imperial National Coarse 3/8-16 with 60deg thread. Some say this size bolt will interchange with Whitworth. That I don't know. I've only used the factory bolts.
Helicoil kits are available in Whitworth 3/8-16 on both sides of pond. Helicoil type insert uses less diameter & tend to hold better than timesert or other sleeve type repairs. More importantly you may be able to preserve the bore for the hollow dowel.
I don't have a hollow dowel on hand to measure its OD. I cannot find spec on the minor diameter of the tap for insert. My hope is there will be original metal left in bore around the dowel so it can locate if even only on the tips of threads.
I've installed thousands of Helicoils. I have a few tips. If possible try to strip out all the old threads before drilling for taping. This makes drill follow hole better. Use a stack of washer as needed & nut on top, screw stud in a few threads, tighten nut & break out threads a few at a time.
Very important to drill hole perpendicular to case. By yourself side to side is easy to align, but front/back often user drills crooked. So have a spotter to look & have you hold drill perpendicular. Same with starting the tap. Keep it straight as possible.
A lot of chips will be generated. Best practice is to separate crank case & clean. If needed you might be able to stuff rags in case below hole & put tape across mouth. Use your best judgment on that.
If you are not practiced with Helicoil repairs you should practice on some scrap alloy. Go to auto repair shop & ask for junk water pump or the like. Drill holes & practice. We have one shot at getting this right & don't want to mess it up.