A gear drive is more accurate than the gear/chain set up. BUT, only at higher rpm's, and only under a load. They're noisy, wear quickly, and are expensive. If you're building a high rpm drag race only motor, it's a good choice. Stick with what you already have, it's perfect.
It's very important that you check valve to piston clearance during assembly. You don't want to be checking that under power. You don't have to swap in checking springs. It's the easiest way, though.
The "problem" is the hydraulic lifter. Without
oil pressure the lifter will collapse and you won't get full lift at the valve. Take the guts out of a hydraulic lifter, and replace the spring assembly with a stack of washers/shims. Then put it all together like final assembly. I set the rocker arms pretty loose to begin with; set it to zero lash and then back off 2-3 turns. That gives me plently of room.
Watch the rocker arm tip as it goes through it's range. That will tell you if your push rod length is OK. Checking push rods are pretty cheap at summit and jegs. They're a very valuable tool.