+1 on the condition of or lack of the umbrella seals. Replacement a good idea at any point if they have one or three missing. Check for and remove missing bits if you can. If all intact best to leave them. New ones in generic gasket kits can be of poor quality and it's not unusual to find that some have split or gone missing when the cover is off for the first valve adjustment after overhaul. There is considerable difference in the B18 and B20 valve/guide set up. Simply put, B18 guides are long and the valve stems are thin whereas the B20 has short guides and fatter stems. The long B18 guides probably benefit from a little bit of oil, especially the exhaust ones. Machining the B18 guides to take the B20 type seals (there are some Ford silicone + spring types available I believe) might stop this bit of lubrication. Yes you will get a build up of burnt oil carbon on the back of the exhaust valves over time and this does have a bad effect on exhaust gas flow. Just how much would be interesting to know, if a flow bench experiment could be set-up. Racers would be thinning out the backs of the valves anyway so that does show how important that area is.