I honestly don't know why you're stuck on a 454. As far as I'm concerned, they are only good 1 purpose these days. RACING. Otherwise they are nothing less than gas guzzling hogs. I'll bet you your Z71 truck, that for the money you get a 454 for and rebuild it for, I can build a small block for less and whip your 454's ass. I have an OEM 454 in my 84 DD. It drinks gas like no other. Its got 3.21 gears, so I get about 8mpg city, and 12hwy, maybe 13 on the hwy if the wind is behind me. Sure its got some torque but its horsepower actually sucks. You get you a 350, Vortec heads, a mild RV cam, a 600 vacuum secondary carb, headers or gasket ported exhaust manifolds and you'll have all the pep you need and good torque for towing amd probably get 2-3 mpg better than the 454. Have you seen the cost of a 350 rebuild kit vs the 454 rebuild kit? Your biggest expense will be to acquire the Vortec heads and Eddy intake for the Vortec heads. Why spend money on a valve job on OEM peanut port 454 heads when its the same cost as having Vortec heads done? And you can use your 90 350 block that you have now. Its probably a 4 bolt main and will have the 1 piece rear main too. Find some Vortec heads and build what you have. You'll be ahead of the game.