Center bolt valve covers started around 87, VORTEC's didn't start until 96 for the V8. 93 is a 193 swirl port head engine.
And yes, your truck is already EFI with a throttle body injection system. What I believe they mean by EFI is MPFI, or multi port fuel injection. Theres nothing really wrong with the TBI setup IMHO. Swap over to a 94-95 big block throttle body, put a 94-95 fuel pump in the tank(94-95 big blocks ran at 32 psi, all the small blocks, V6's, and big blocks to 93 ran at 8-13 psi), cam swap, set of headers, 3 wire O2 sensor, open exhaust, port your current heads and intake, and it will run like a COMPLETELY different truck. Next weak link will be your trans, so save up for it. I have yet to not have one lose a trans after I've done an engine for one. The amount of off idle torque the swirl port heads can make is RIDICULOUS! But it will NEVER be a speed demon unless you want to put a set of heads on it, and spend some money on it. Swirl port heads work great for stump pullers, RV's, and airboats.