It is possible to make a vacuum secondary 600cfm Holley work on a
AMC 258 i6 / 4.2l . If that's what you want to do, go ahead and do it. I would suggest using a 4bbl intake manifold rather than an adapter since the adapter will cause an unnecessary restriction in air flow while the 4bbl manifold will not have that restriction.
I suggest that a 600cfm is vacuum secondary only because the
AMC 258 i6 / 4.2l will never use all 600cfm. The secondaries will never fully open since the
AMC 258 i6 / 4.2l doesn't draw in enough air, even under heavy load at WOT. It's more than you need but the vacuum secondaries let you tune it to work just fine. After all, GM built 305ci V8s with a 795cfm Q-Jets at the factory. They just tuned them so the secondaries never fully opened and they ran just fine.
I'm going to suggest alternatives and why I would personally choose them over that 600cfm. Feel free to ignore my suggestions if you like. I don't want to come off like I'm trying to tell you what to do. I just want to give you information so you can make an informed choice. I won't be offended if you don't choose what I would choose.
One consideration against the 600cfm Holley is the off road performance. The basic 600cfm Holley is terrible off road, as is the Edelbrock. There are other, much better alternatives out there. But if you don't go off road, it's not an issue that you need to consider.
As an alternative to the 600cfm Holley, the 470cfm Holley Truck Avenger would be a really good choice. (There's no 570cfm TA and the 670cfm is way more than you need). It's great on and off road and still enough carburetor for the
AMC 258 i6 / 4.2l . I know several people who run that 470cfm on a 360 V8 in a full size Jeep and have plenty of power through the usable RPM band up to 4,500-5,000RPM. With a little tuning, it will give you all the performance you need in a
AMC 258 i6 / 4.2l . And with the smaller primaries, it will be better on gas when you keep your foot out of it.
The other alternative, the Motorcraft carb comes in a couple of different CFM ratings, depending on which engines you get it from. I don't know the CFM ratings of each carb but I do remember that the body of the carburetor determines the flow and they have a casting number on the side of the carb body that tells you which is which. The Ford 302 or
AMC 304 typically use the 1.08 casting. That's the one you want for the
AMC 258 i6 / 4.2l . The Ford 351 and AMC 360s typically used the 1.21 casting. It's harder to get that one tuned down for a
AMC 258 i6 / 4.2l . Possible, but more work. I've never looked into doing the swap myself but I know there is a lot of information about that swap out there.
Disclaimer: I'm quoting those casting numbers from memory. If I got them wrong, someone please correct me.
I suggest the 2150 for several reasons. First is that it's a good carburetor for both on and off road use. It's very consistent at all kind of angles where a lot of other carburetors will lean out or bog down. Second is that it doesn't need constant tuning like some other carburetors. I haven't touched the 2150 on my J10 pickup since last spring when I rebuilt it. I still drive that truck to work on a regular basis. It just starts and runs every time. I may get in there and adjust the choke a little when winter really sets in but only if it really needs it. Third is the altitude compensator built in to that 2150 really does work well. A friend of mine took his J10 pickup with the 2150 from near sea level in Texas to Colorado, over a 13,000+ foot pass, and back to Texas without ever having to touch his carburetor at all. It just kept running and running. The altitude never stopped him once. All of the guys with Holley and Edelbrock carburetors were having to stop and change jets every time they changed 2-3,000 feet in elevation. Fourth, you can swap it on to a
AMC 258 i6 / 4.2l without buying a new intake manifold.
You can even buy a remanufactured 2150 at most parts stores. Just be sure to check it over. The previous owner of my J10 was frustrated trying to tune in the reman carburetor on it. Turns out they didn't put in the gasket on the power valve and it was leaking fuel. Once I rebuilt it myself with all the gaskets and got it dialed in, it's been running great ever since.
