Grab a van with a 4.8 and an 80E. Swap it all with a Walbro 255.
Van donor - about $1000
Tune - about $300
Later after your sick of 300hp add turbo and the niceties that go with it for $1500-2000 and make 600 whp. When and if the motor goes kaflooey then spend $250-300 on a 5.3, or stick with the $100 4.8's.
FWIW, there are two places locally that will sell running 4.8's on a pallet for scrap value ($100). 6.0's on the other hand - $1500.