Hi, these old engines are not known for real neck snappin off the line pick-up, the HP for your Desoto engine would be for a 251 ci, about 116 hp @ 3600 RPM, compression ratio 6.6 to 1, but they are very torquey long stroke engines, bore = 3 7/16, stroke 4 1/2 inches.
These engines get max torque at about 1200 or so RPM, so just off an idle they have there torque punch.
Now with just this stock engine, and usually the Desoto may have come witha 3.73 rear end or a 3.54 read end if it had the 4 spd tip-toe semi-auto trans and fluid drive coupler. It should cruise all day 60 to 65 mph, probably could do 90 mph.
The speed equipment is out there, probably turn this engine into a 150 170 hp engine, but building a 250 to 300 hp engine, ain't gonna happen.
Now what trans do you have in this puppy now, does it have a fluid drive coupler, any idea what diff ratio you have.
These cars with the flat head engine souped up, and a T 5 trans will get you going nicely, the flat head with dual carbs and exhaust sound and run great, fuel economy is not bad too. Some guys in your neighbourhood, Dublin GA, have even installed AC too.
You have 2 great members of this forum right in your own state, try and get to know these fine Gentleman, they are a wealth of knowledge of old Mopars.
Here is a website that sells the disc brake kits, there is install info as well, www.rustyhope.com. Swap mees, forums like this, ebay, will help in your search for trim, there is a 48 Desoto, 3 miles from where I sit, it is in the bush but high up on blocks, near a river delta, but the owner won't sell parts, too bad cuz there are a lot of nice parts on the car, so there will be more all over the USA and in Canada, where I live.
Good luck, keep us posted, this forum, as as well as a few others will bea of trmendous help to you with this car.......