I think it kind of depends on what style of car you have. When I restored my 28 coupe, I installed the cowl/gas tank welting before painting the car. The welting between the cowl/gas tank and A pillar should be body color. I installed the black fender welting after the fenders were painted, since both are black. I painted my car after I installed the roof material in case I damaged the painted area installing the top (fairly involved on a special coupe). This are the way I did my car and it worked well for me, but others may do it another way and have good luck. Hope this helps.
Rusty Nelson