Using a stock TBI Intake Manifold

When using a stock throttle body fuel injection manifold on the older small-block Chevies, you have to keep in mind that in 19?? GM modified the heads so that the center 4 manifold bolts are more upright instead of as the same angle as the other 8. This allows you to actually get a wrench on them and tighten them properly. So, on the TBI manifold those 4 center bolt holes are at the wrong angle to be able to bolt it down the heads.
Here you can see the holes that need to be modified. They are at the wrong angle to be able accommodate the hold-down bolts.
Using a rotary file, carefully elongate the holes to allow clearance for the bolts to enter the heads at the right angle. It took me several tries to get the holes the proper size. It's easier to make the holes a little bigger each time than to try to make a too-big hole smaller, if you get my drift.
After the holes are the right size and you are trying to install the bolts, you will find that the surface the bolt is supposed to tighten against is at the wrong angle. That means you'll never be able to get things tightened up right. What you need is a little wedge-shaped spacer to make things right. You can make them yourself from scratch or you can modify a TBI spacer intended for a different use. Here are a couple of views of spacers that are available from Professional Products that are meant for use on one the intake manifolds they sell. Their part number 52110, Adaptor Bushings. They are about $5 plus shipping.
Unless you have a machine shop at your disposal to make the holes in your TBI manifold just right, you'll have to modify these bad boys by cutting off the lower boss, leaving you with a nice wedgies as show on the left.


Here are a couple of views of what they look like installed. Please note that it would be much better to pick up some bolts that require a smaller wrench. The ones shown are barely serviceable.
If you'd like to make your own wedgies it is fairly easily done. Here are ones made of aluminum which was crudely cut on a band saw. An 18° angle is about where you want to be. These work perfectly and cost is very minimal.