Get aluminum. Plastic will leak again. Like Hound said, probably won't have to drill.
Aluminum 4.2 cover, tap two holes on the drivers side of the block, buy new bolts to replace crap that comes with 4.2 cover, install.
A tap is a T shaped tool. Easy to use. Hole is already there, and a tap looks like a screw. You screw it in a little, back it out, screw it in a little, back it out, until you hit the bottom of the existing hole. That threads the hole. It's that easy. First tap I ever used was putting a 4.2 valve cover on my green Jeep. I was shocked at how easy it was. Some things sound scary but they're easy.
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