I wouldn't use more than 7# on an original 58 rad, which I think is what the manual would call for, and the flip top type just gives you a safety when removing the cap hot, they don't 'seal better' - you shouldn't even need to have the cap on tight for it to stay at the proper temp if the cooling system is all good - also do not overfill the rad, it should be down at least an inch or it will push out the excess, or with too high a rated cap, will crack a seam in an old rad
with the laser, check the thermostat housing for the "high temp" and check the lower rad hose or it's input at the water pump to see how much the rad is cooling, should be at least 10-15* difference - check the rad top and bottom tanks too - the stat housing temp [when warmed up] should be close to the stat rating, and stats can go bad or be defective out of the box .... can also check around the block to see if there's a hot spot that might indicate a plugged passage
Bill
