The casting number on the head will lead to combustion chamber size. I general most everything pre 1987 back to 1971 were 76 cc open chambers of low compression and slow burn intended to decrease NOx emissions.
The other big contributor to compression ratio is specific head gasket thickness, since there are many to select from that vary from as little as .015 inch to as much as .053 inch and diameters vary bore size it is necessary to know the gasket be maker and part number.
The 98 block uses a one piece seal crank which used a different bolt pattern to the fly/flex, the older crank with 2 piece seal can be used with an seal adapter.
Inside the bearings are the same, rods same length at 5.7 inches, the pistons at same bore but have some detail differences but years can be crossed. Piston crown volume varies between light duty trucks which will be the same as your 86 pistons, heavy duty truck engines use a deeper dish to lower compression but the L31 chamber volume of the heads stayed at 64cc. Piston to deck stayed at .025 inch
One cylinder being high to others low sounds like either ring sealing problem or miss adjusted valves, or one cylinder tested wet with oil while the low ones remained dry. Your older flat tappet will work with the older timing set and no roller cam thrust plate, you need standard Chevy flat tappets and the flat tappet push rod of 7.8 inches. You need to use the older unguided rockers, not the L31 self guiding or you can get binding of the pushrods.