Originally Posted by SoK66
Read an interesting old JP article the other day about the 2100. How difficult is it to adapt to a 258?
It's a well documented and fairly uncomplicated process. There are several articles and posts and documents floating around about it.
You need an adapter plate to connect the 2100 to the 258 intake manifold. These are low cost and easy to find. You need the correct gaskets which are also easy to find.
You need to buy or build an assembly to attach the throttle cable. You may need to install a bracket and throttle cable attachment stud on the throttle shaft on the 2100. One consideration is do you want the cable to pull down or pull toward the firewall.
You may need a grounding wire if the automatic choke on the 2100 has a grounding tab.
One tricky area is the air filter housing. The standard air filter housing may not fit right (I don't remember the standard size off the top of my head)... 11" or 12" maybe). It might bump into your brake booster, if you have one. I ended up using a Moroso 9" housing and filter that works well for my 85' although I am roped into using their air filter replacement part.
This is the guy on Ebay who sells complete 2100/2150 kits for the 258:
You will typically pay $300+ to him so for a complete kit with a rebuilt carb, adapter plate, bolts, throttle linkage parts, etc. If you can get a 2100 or 2150 for cheap and rebuild it yourself, you can do the whole project for less than what this guy charges.
The benefit of the Ebay guy is that you make one purchase and wait for the big box to show up. He is of course selling used/rebuilt carbs, so in my experience you can expect some variation about what specifically you will get since there are a lot of variants with the 2100 and 2150. Another issue is the problem of play in the throttle shaft. When you're dealing with used carbs, you always have the potential problem of a throttle shaft that is loose and allows a vacuum leak. I would pin him down on this issue and do all you can to ensure that you're not getting a carb that has this problem. His install documentation is OK but a bit unpolished.