Im in the same boat mines an '89 350/TH400 4.10's, luckily its my shop truck i dont currently tow with it but haul a good bit. Axles, engines, dump runs, lumber, firewood, topsoil, mulch etc. Mine has 36" HMMMV rims and tires which i know contribute the most to the lack of power. I can watch my speedo go backwards going up a decent hill empty. I usually have to pull to the shoulder and let the line of cars pass. I just picked up a fairly complete,no front accessories but everything else, '99/'00 454 Gen VI that I am going to rebuild to put in it hopefully this time next year. Probably convert it from the MPFI to TBI cam swap, headers etc. I had this same motor in a '99 2500 Sub and it had plenty of power I know I will lose some if I ditch the MPFI for the tbi.