I've used that three-part tape many times with great success, but it works best for straight lines. Several places on the Model A the pinstriping curves (e.g., between the top and the section behind the rear windows, or on the inside of the gas tank on deluxe models) and that three-part tape doesn't do corners that sharp very well.If you want to try and do it free hand, you can put a wide piece of masking tape in the area that you want to put the pin stripe and practice on that. I thought that I would never be good enough. I bought the pin striping tape that has three sections. After putting the tape down, you can pull up the center section that leaves a clear area for the paint to go. Different widths are available. At the ends I pulled up on the brush (I used a pin striping brush) so that the ends looked like the pin strip was done free hand. After the paint is dry, you pull up the other two parts of the masking tape. The pin striping paint comes off easily with acetone, so if you make a mistake you can erase it.
Any good auto paint supply place should have it. The pinstripes were nominally 1/8", but the Paint and Finish Guide notes that in practice they were more like 3/32". You could lay out your pinstriping using this thin masking tape that I found on Amazon, mask right up to the edge of that tape, then remove the original layer and paint. You may get a slight raised edge where the masking tape was, so you may need to do a slight rubout of the pinstripe.NK where did you purchase the tape with the 3 sections?
I used a self employed professional who works on high end dealership and autobody repair cars requiring striping. I gave him my painted gas tank to do the striping on the inside before assembling the painted body. After I completed assembling the body he came to my house and did all the required exterior areas. This sounds like it would be expensive but it wasn't and he did a super job all free-hand. I would never attempt to do striping on a newly painted body.