Need to install new jets in Holley #4150. I believe these are the main jets? (The mains go behind the float bowls correct?) Is this simple enough or are there any special instructions?

When you pull the fuel bowl off, you'll see the jets along the bottom of the metering block

Funny, I just came up from the garage completing that very task! You either want a little container to snake in under one of the lower bowl bolts OR put a lot of rags under the carb to catch the approx 3 ounces that will spill. Loosen all 4 bowl bolts a little to relieve tension. Remove one of the lower ones and let drain.
Remove the fuel inlet line at the bowl. Remove remaining bolts. Carefully remove the bowl while assiting the removal of the gasket. If your careful and you have the right type (so it does'nt tear) you can reuse. Remove the jets (reg screwdriver, no big deal) install new and reverse process. Torque bowl bolts to 25 inch pounds-not much. Fire car (FIGURATIVELY SPEAKING) check for leaks

Let me add one more note, when putting the bowl back on, make sure to flip the accel pump arm down (bottom of bowl)so that the spring loaded bolt will land on the top of this arm "before" you bolt the bowl back on.

Get a Holley Quick Kit with the blue nonstick gaskets. Then if you tear a gasket you have one handy and the nonstick gaskets are good for a long time. Or buy another reputable manufacturer's nonstick gaskets.
R.

when you pull the bolts out of the bowl, pull One top one first (for venting), then both bottoms. Hold a small can as you pull out the first of the bottom ones. let the gas drain out the bolt holes into your catch can (whatever will fit into that spot, I use a small tupperware). This will help get the job done without spilling gas all over the intake.