243 heads milled?
as for how much you can mill i have no idea but most will tell you to measure. stock gaskets can be used regardless of the amount milled as long as you have enough clearance(ptv) and they are for the coorect year (98's were different i believe). pushrod length will depend on the correct amount of lifter preload for the setup. most will tell you to measure as well. sorry if i wasn't any help, i'm no expert. good luck


