Long distance cruising on my 1990 Carver 42 ACMY

Once you get to Panama you still have quite some motoring to get to the Great Loop.

Any coastal passage is possible if the weather is calm. Doing a run like that in that type of boat will require careful planing and execution.

You might have better success finding someone who has done it if you can join an Owners group dedicated to your type of boat.

If you do it take your time and fish along the way. I have been up and down that way a few times and always had great fun filling the freezer.

Is it possible? Yes. However, as K1W1 says you'll have to do careful planning and pick your spots to cover some of the coastal areas. The second thing I'd point out would be it will challenge the condition of your equipment and your skills in maintaining everything. With coastal cruising you're talking over 5,000 nm from San Diego to Brownsville, TX. I don't know your speed and range you'd expect. But assuming 10 knots, that's 500 hours.

A couple of other things I'd toss out. On top of fuel range which will probably be a challenge to you in a couple of areas, you're talking some distances that may require more overnight running than you're use to. I'd definitely want enough of a crew to be able to switch off and not exhaust anyone.

We made that run early this year, but not in the size boat you're talking about. What is your speed and range?

Range is the big question. You should be fine on the Great Loop for range and seas. DK about coming around from San Diego. What's your distance between ports in those section? Pick your weather carefully for those ocean and Gulf legs. I heard there's some spots where it's a long way between safe harbors. For two sections of the Loop you'll have to get your hight down to 19'1".
P.S. On overnight legs having enough crew to have 2 on or to do short shifts, then someone the run the day shift. Sleep sneaks on like a thief at night on the water.

As you figure your fuel burn keep in mind that any kind of seas will affect that number negatively. So you want to maintain a wide margin of error in your figures.

You should actually try to measure your fuel economy by running a distance starting with full tanks and ending by filling the tanks and calculating. There is a big difference between 8 kt and 9 kt. I have twin Cummins in a 37 Tolly, weight 22000, at 8 kt I get 2.3 Nmpg with both engines running. At 9 kt I get 1.3 Nmpg. Heavy head seas have reduced my 2.3 Nmpg to 1.6 Nmpg. Lastly your usable fuel volume is about 20% less than volume state on your tanks. This is due to the air for expansion on top of the fuel and the suction is anyway off the bottom to prevent water from entering the engines. I have taken my tub all the way to Cabo, I sometimes take plastic fuel barrels to compliment by 240 gallon usable tankage but only need it when I stop and do a lot fishing along the way.

Personally I would love to make that trip but have not looked at fuel stops below Cabo.

Below Cabo the only long run we found was from Golfito to Panama City, about 400 nm. There may be available fuel between the two but I'm not aware of it. However, on the Caribbean side we went much further between stops and don't know if there was fuel we weren't aware of. Specifically between Boca del Toros, Panama and La Ceiba, Honduras. We weren't in need so didn't search more carefully.

Based on this and your other question about fuel mileage it seems you are not an experienced boater. What you are considering is something only an experienced boater should consider that knows his/her boat thoroughly.
Your Carver boat can probably take more than you can weather wise but is not considered an open water boat.
The loop is safe and easy to cut your teeth on.

Consider shipping your boat to the US east coast or better yet selling and buying in Florida. Used boats in Florida are numerous and probably cheaper than the west coast.

Excellent point.