Body lift wont affect brake lines/drive shafts. The steering shaft will need to be addresed depending on how much body lift you plan on going. I would start by ditching those front lift blocks as soon as possible for a b52 swap or a good set of lift springs. I'm not a fan of lift blocks at all...
I had the same issue on my '85 burb. I took a cutoff wheel and cut the old rusted bolts of and replaced the sway bar mounts with red energy greasable bushings. The kit comes with new bushings and mounts with grease fittings. I got them at autozone off the shelf for about $20.00 and picked up...
Im runnin 33" tires now so a prime gear swap would be a 3.73 would geat me close to the same as stock tires and 3.42s reason for 4.11 is im gonna run 35s next and prime gear for 35s is like 4.05...thus the 4.11 imo
Im running a 454 with a stock q-jet 3.42s and 33's th400 i get 8mpg im hoping to gear it 4.11 and see if it helps. I was getting 10 with 31 10.50 15. But im also driving a suburban which is heavier than your pickup.
Old77 did a write up on his NBS seats in his truck. http://www.gmsquarebody.com/forum/showthread.php?t=375 it might offer you some info. Should be easier in a blazer. Cause the floor is flatter than a pickup without the recess for the dual tanks.
I had a slow leak in mine also. I tried the stop leak it worked for a week then came back worse. I soldered it but it sprung a leak elsewhere in the radiator. If its leaking in one place than more than likely have other weak spot waiting to go. I changed mine with a new one from oreilys auto...