The hydromatic was a good transmission. I have a version of that vehicle with a dump bed and a 4 speed with a pto.
It seems odd that you want to race a truck that big, especially with a 283, unless drag means drag a trailer behind it in which case you may well want to keep your rear end gears and obtain an overdrive for it for more comfy freeway driving and fuel economy. With that trans you may be able to bolt on an aftermkt overdrive. Mine has the drum E brake behind the trans so I would need a stand alone overdrive (cost about 3k).
Good luck trying to find higher gears for that rear end. There are alot of guys in that same boat and finding even a 4.11 is hard. The original in mine is a 4.57 and believe me, at 50mph that motor is screamin'. To help mitigate the problem I added 19.5 inch wheels which raised my tire height from about 29 to 33 inches. That helped by about 10mph or a few hundred rpm's. This places me at roughly 2600 rpm's at about 60mph. I'd like to get closer to 2k for sound, comfort and fuel economy.
Now regarding brakes. Good luck. There is no off the shelf disc brake kit for a 3600. See the discussion regarding this in the hipo section. When I have this solved I'll be letting everyone know.
Mine was ugly but now its...less ugly.
