Let's start with the assumption you just want your question answered. IF it's the original engine, it's straight valve, and a Silver Diamond 220. The simplest way to know if it's straight valve, is to look at the valve cover. If it has a raised ridge running lengthwise, front to back, it's tilt valve. If it only has the six raised ribs running side to side, it's straight valve. If you aren't sure at that point, pull the valve cover off, since it's coming off anyway to change the head gasket. If the valves are straight up and down, as opposed to angled, then you know for certain. If you aren't sure it's the original engine, you probably should determine just what you have before going much further. The L series didn't have a 100 (light duty half ton). The 100 series started with the R series. The L's had the 110's (110, 111, 112) and the 120's (120, 121,122), etc. So you probably have a 110 series. Silver Diamond engines were all straight valve. Next came the Black Diamond series, and some were straight valve and later tilt valve. Assuming you still have the data plate in the cab, I think on the kick panel under the dash on the driver's side on the L's, check the engine number against what's on the engine. Here's where you'll find the pad with the engine numbers. Presumably SD220 and the serial number.
Or post a picture of the driver's side of the engine and the valve cover, or the engine number from the pad and we can tell you what you have.