Primer
Wagon, the 409 distributor is shorter and larger in diameter than the BBC and the SBC. It won't seal off the passages. So the oil won't go where it is supposed to go, like up top. The Show Cars primer is the right size in length and diameter. I have a BBC primer I used on them. I can prime the bottom end and the oil filter, but nothing goes up top. The Show Cars, being the right sizes, is your best bet. It seals off the passages, and forces oil up top like your 409 distributor will. Money well spent. Alternative is to have someone make a sleeve to go over a BBC or SBC primer to the 409 dimensions. Machine shop work. I think that's what DonD did. It will work, also. You would have to take your 409 distributor and a BBC in to the shop so they would have the dimensions to work with.
Hope this explains it to you.