I recently swapped my 97ext 4.3 v6 to a 5.3 carbed, and I kept my stock fuel pump used 2 regulators to bring the fuel pressure down to 6psi carb. Installed a .500 lift cam rv pulling cam better than the measly .466 lift the stock 5.3 has. Has massive torque down low to about 5k rpm, I spent $1500 on the whole setup, carb, intake msd, 5.3 was $150 long block with bad cam bearing, I installed the new cam bearing they were $20, 97 0411 using the pcm to control the 4l60e. I used a afr guage to tune the jets on the carb I went up a couple sizes not hard to tune at all Just my expieriance . I would also definitely consider a full 383 kit from ebay, my buddy has used them before with success, comes with everything you need for less tha $1000
Not bad for 400hp.