Re: New to this and my 1st Question
There will be an advertised towing weight for your truck. Being a 1/2 ton, it will probably be about 7000 pounds, but you need to check to be sure. Adding the transmission cooler may up it a little, and if the engine is bigger than the original engine, that may up it a little too. Since it did not have the transmission cooler, it probably is not set up for towing and will have low gears in the rear end which are poor at towing. So your advertised weight might be only 6000 pounds.
However, this is with absolutely nothing in the truck (no cargo, no accessories, no passengers, no animals) except a 150 pound driver and a bit of gas. So you need to subtract the weight of anything additional in the truck. If you don't have an idea what this is, then 1000 pounds is usually a safe guess.
Also, the towing weight is for a flat bed trailer with 'no' wind resistance. Take about 80% of the (towing weight minus the truck load) to get your 'true' towing weight. For most 1/2 tons, this is about 5000 pounds.
27' fifth wheel will probably be more than your truck can handle. I had a 25' ultralight, and its GVWR was about 7200 pounds. Loaded, it was about 6000 pounds, which was more than my 1/2 ton could handle, salesman's promise to the contrary. Using dry weight when matching a truck and trailer is risky, as the dry weight does not include any changes make after it left the factory (dealer or previous owner), and towing an empty trailer is of limited value. GVWR is the safest, or 'loaded weight' if you know what you will always be carrying.
With a 1/2 ton vehicle, you not only have to worry how much you can tow, but how much the trailer puts on the rear axel. A fifth wheel usually has 10 to 20% of its weight as 'pin weight', which goes directly over the rear axel of the truck. Say you have a 6000 pound trailer; that means that you are putting perhaps 1000 pounds over the rear axel, which often will exceed the rear axel maximum weight. For a 1/2 ton, a travel trailer, with its lighter pin weight, may be a better choice.