Hi David,
Depending on what year the HEI is the spark plug gaps were .060 when they first came out on Chevrolet engines but were later reduced to .045. .035 is a bit too close and will not burn hot enough, so I would stick with .045. The only difference you will find for water pumps is either a short or long. The short pump will be very close to the timing cover. All the other items such as alternator, PS pump are all going to be the same depending on whether it is a short or long water pump.
The basic design of these small block Chevy engines are all the same except for the mounting surfaces on the heads for the alternator. It is pretty easy to notice the three mounting holes on the front of the heads. Your engine should be a two piece rear main seal. All the flywheels from 1985 back will interchange except for the SBC 400 which was external balance. Late 86-up was a one piece rear main seal and took a different bolt pattern for the flywheel. From 85 back they used two different size flex plates (flywheel), one was 168 tooth and is considered the large flywheel and will take a starter with a offset bolt pattern. The 153 tooth flywheel required the starter with a straight bolt pattern.
For all practical purposes, there are very few items that won't interchange on the 85 back SBC. On the timing, hold onto your hat.
SBC Chevrolet engines like a lot of timing. Most of the time you will find that the book calls for around 8* BTDC (with distributor vaccum disconnected and plugged- RPM around 700), I typically run around 10-12* BTDC. The engine is a lot more responsive and better all around performance. IF I run into the engine kicking back slightly when trying to start or hear spark knock (pre-ignition) I will kick it back a slight bit. This is by no way what the book says, it is just my own personal experience.
