Increased power for Chevy 3500 Class C

The 350 of that vintage is a L31, Vortec 5700. It came in power ratings 255-350 HP.

The contemporary 454 was a L29, Vortec 7400, rated at 290 HP, with about 20% more torque at low RPM, thus it makes more power at the running speeds you probably find more comfortable. But it doesn't make that much more power than the 350 at peak output, because the truck build of the 454 doesn't run as high an RPM as the small block can.

You can get a fairly decent gain with the 350, maybe 60-90 HP, by replacing the intake manifold and injectors with those pulled off a L31 built for marine applications. Those engines were designed for continuous high output at high rpm, rather than flexibility over a wide range of RPM.

There are more ways to build it. The truck motor you have is a popular starting point for performance builds of Chevy small blocks, because those Vortec heads are great. Earlier truck engines needed expensive aftermarket heads for significant power gains, with the Vortec good heads came free from the factory. At the 350 displacement, it is not hard to build to 350 HP, and stroking to 383 still gets you a reliable engine for truck service.

You have to understand what you are doing. To get more power, you improve air flow through the engine so that it gets more air (and fuel) at high RPM, raising the torque peak to a higher RPM. The tradeoff is that the larger flow passages reduce velocities at lower speeds and power settings, costing low end torque. Which is why for a truck you build maybe for 300-350 HP, rather than 400+ (certainly possible, Cup cars get 700 HP out of a 350 by running 7000 rpm).

A lot depends on what you mean by "struggling." If you don't like that the transmission is downshifting, and the engine running 4000-5000 rpm to get up the hill, these upgrades of the 350 don't work for you, because they are about improving the performance at higher RPM.

Chevrolet Performance sells a number of different truck small blocks as replacements for the pre-Vortec 350, which was only 160 HP, not the 255 you have now. Most bring performance up to the level of the Vortec, options that go beyond are sometimes not street-legal, particularly going back to carb motors on vehicles that were multi-point fuel injected and computer controlled to meet emissions standards and still have high levels of performance.

After pulling my 5th wheel for the first time I realized I needed more power. I had a 350 swapped for a 454 and after spending $2500 to make the swap and get out all of the bugs (bad vibration damper, fan clutch and new 4 row radiator, burnt spark plug wires) It really didn't make that much of a difference...sure I was able to go up hills about 15MPH faster while towing.

On a 6% grade that meant I was doing 43 instead of 30.... whoopee.

After awhile, to save wear and tear on the drivetrain, I learned to just keep it in first gear and cruise up 6% to 10% grades at 22MPH and enjoy the scenery. Often I'll feed the squirrels as they run along side. A real bonding with nature experience.

Now I've gone to a Duramax Diesel and Allison trans equipped truck....made all of the difference in towing up 6 and 10% grades....This truck more than meets my needs.

Moral of my post.... Get a more powerful MH.

IMHO you would be wiser to yank the 350 and install a 4 bolt 350 truck motor built up. If you already have the 4 bolt truck motor, pull it havd have it built. 3 to 4K to get it built vs 8 to 10K to find suitable motor and trans and then trying to shoe horn it into the chassis seems a no brainer to me.

Engine Swap Cautions:

1. Be sure the 350 and 454 use the same CHASSIS. That's right, Chassis. I do NOT know about GM, but when we had a Dodge, I went to replace the anemic 318 engine. What I wanted was a 383 big block but wound up with a 360 small block. A man who had worked for Dodge told me "Good Thing - They used different crossmembers for the Slant Six, the Small V8, and the Big V8.

2. Particularly if you have to emissions test, you're on track about Donor Vehicle. It is VERY possible that not only is the Engine Control Module different, the wiring harness might also be different. This may go deeper than you want to cut.

Counterintuitive that the 350/5.7L will fall short and the new 6.0L does a decent job, but true. It's a different engine, not just 0.3L displacement.

If you want to get radical, you can Stroke your 350 to 6.2L (377, 383, something like that. Also years ago there was a 400-CID small block. I think it was a special block, though, not something you can create from a 350.

Any case, you'd have to allow for, or be sure the ECM allowed for, greater fuel demand from the larger engine.