You probably won't have to drill any holes. On my 85's the holes were all there, I just had to tap two of them on the driver's side of the engine. I went and got shorter bolts rather than drilling the holes deeper, because I was worried about drilling into the water jacket. Been on there since 2002 and not leaking a drop.
I'd take the plastic one off and look before you get too excited though. 258 heads were different over the years, and some had the holes there and some didn't.
Should be the stock mounting bolts on the passenger side of the engine and the front and rear of the engine, and then two holes on the drivers side. IIRC I ended up buying all new bolts because the ones supplied with my "4.2" valve cover weren't the right size, but all I had to do was tap the two holes on the driver's side and it bolted right up.
85 CJ7 258, borla header, Team Rush, MC2100 ,optima batt, Warn 8274, agr super box 2 & pump, T5 w/ B&M shifter, centerforce 2, superior 1 piece axles, warn hubs 2.5 in lift, 31" BFG MT
85 CJ7 258, A/C, T176, MC2100, Team Rush, Warn XD8000i, superwinch hubs, 3in lift 31" BFG AT/KO