I wouldn't worry too much about that. Lister used big rocker arms that rotate through a small arc with little lever action with pretty static geometry. Also, the clearance adjustment is on the pushrod end, so it effectively changes the length of the pushrod, and not the fulcrum point, as is the case with many car engines.
In short: If you fix the exhaust lifter problem and set the valve clearances to factory spec, you should have no problems. A double-check is that the rocker arms should be pretty close to horizontal when the valves are closed. If they're not, you have some altered geometry somewhere and should investigate. It's all adjustable and repairable.