Engine swap

has anyone put in a v6 in a 64 G10?

4.3 would be physically easy-peasy because it's 3/4 of a SBC......I just don't know why anybody would want to do that.... A 4.3 is 262.4 cubic inches...… A 292 is much more stout and simple and fits in a first gen without cutting the sides of the box (like a V engine requires)
A straight six with Clifford research stuff on it is much cooler...... Or even better, a Pontiac OHC six....

Not as hard as it may seem. I put a SBC in my 1965 G10 Van in 1977. Take the 6cyl and trans out. Cut a the engine cover middle of each side. Cut upwards on the side of the cover and bend up to a right angel. Cut the floor back to the frame rail. Then bend the floor upwards to meet the upper panel and make a box like structure. Take some sheet metal and you can use a welding torch to add to for a box. Gas weld, solid to the floor. Never take the engine cover out like everyone else is doing, take too much strength out of the floor. You can make a removable box and blt in but it would not be as strong. The same engine cross member for the 6cyl will work for the V8. Just remove the 6 cyl mount and you can bolt a v8 mount to the Cross member. You can use the same shifting linkage you need to link to a 350 or 400 trans. I have a 350 V8 with a 400 turbo hydromatic in my 65 since 1977.

Why hasn't anyone put a vortec atlas inline 6, or even the 5 cylinder in one? That seems like the way to go to me.

So have decided to stick with the 250 6. Has anyone put A/C on one of these , Might be room enough for condenser.

The GM 4.2 6 cyl may be a good candidate- they were rated between 275 and 290 hp, a company named Lime-swap will make your harness easy to wire for a swap, I would just take the complete engine/transmission out of a trailblazer, send the harness out for a rewire and fab some mounts. A Local driveshaft shop can make a driveshaft for you- use the trailblazer output yolk and the yolk to your differential with a length measurement. I have done a few swaps in other vehicles so this is really not a big deal. If you cant weld or fab stuff then this is not for you. You could stick with the old stuff but installing a modern drivetrain makes such a huge difference in driveability. You can drive cross county, leave it stored for a year and a simple turn of the key and its running. I did a Camaro LS1 engine/trans into a 64 Dodge A100, 320hp and drove so nice. Did a L67 supercharged 3.8 swap in a 87 mitsubushi van- 300+ hp and people have no idea what just happened when you just passed them. The 4.2 is tall with the overhead cams you may have to raise up the engine cover.
As for the 4.3, they are getting high mileage, the acessories stick out the front really far but you can use a non computer controlled 700R4. Only looking at 175-190 hp though, 4.2 straight would give you 100hp more. Now a gm 4.8 swap, they are newer, still in good shape and cheap, tons of support on harnesses and mounts as they are the same dimensions as the 5.3/6.0. The stock radiator won't handle much without going to a 4-core or a larger which would require more fab/cutting work.