Well, depending on what year it is, it really really will make a huge difference on if it is a "good" motor. It sounds to me like it's carbed which is leading me to believe that it is a pre-fuel injection motor. I would personally suggest NOT converting your truck to Carb, particularly for towing. You'll get much more control and efficiency with FI hands down. I also wouldn't recommend the TH400 in your tow pig - not having overdrive with a 454 is fiscal suicide. A 4L80E or NV4500 would be much better. a NV4500 is the same length as your current 4L60E so you won't have to change crossmembers or drivelines. The TH400 and 4L80E are both different lengths and would require custom drivelines & moving the crossmember. All of the trans options will require installing a 32 spline input shaft in your current transfercase - which is plenty strong behind a 454.
Most 454 motors had lower compression heads (around 120cc chambers) and small "peanut" ports. Peanut port heads are efficient, have low end grunt but really flow like crap (anything above 2k RPM). Almost every carbed truck motor and all TBI motors had peanut heads. There were higher performing heads on many of the cars, but many of the higher flowing rectangular and oval port heads were higher compression and at that lacked as serious of a low end because of the higher flow.
Honestly I would try to find a Vortec 454. They are MPFI and really respond well to larger injectors and a cam, and being OBD2 are really easy to tune. The block is also the Gen 6 variant which has a good amount of design changes that enhase it, such as 1-piece rear main, larger oil pan, aluminum timing and valve covers, and a factory roller cam. They also had one of the better all around head designs - incorporating a small 100cc chamber (higher compression) with a nice mid-size oval port intake. Give good low end and can still rev nicely.
I was going to drop a dead stock TBI454 in my truck. Originally I though I was getting a old 1990 Mark IV TBI 454 (same as the older carbed motors) but someone had replaced the original TBI with a new GM replacement in 1996. I lucked out and ended up with a Gen 6 GM service replacement motor - but it was fitted with TBI trimmings. It had peanut port heads, boat anchor EGR intake, and the lowsy flat tappet cam. After realizing what I had, I essentially built a Vortec from it (crank is the same, difference is in the heads/intake). So I scrapped my peanut port heads for Vortec ones, installed a OEM roller cam setup with a GMPP Mercruiser Marine FI cam and an edelbrock dual plane intake. I also worked over my TBI unit to feed it. Based on the Mercruiser Vortec 454 motors (which is what mine now is, except it's fed with a TBI instead of MPFI) I am sitting around 385HP 500FTLB and still am able to get 14MPG highway unloaded and about 8-9MPG towing.
