Ilford and Kentmere make 9,5 x 12 " paper, which would work perfectly, but I haven't seen it in B&H or Adorama. Not sure if it is obtainable in the USA. It certainly is in the UK, from Ag Photographic, Silverprint etc. It is locally available in South Africa through specialist importers, which is where I get my stock. The way I cut my frames for 6x7 is 2:3:3:2, and I leave the rebate on to make it easier to handle the end strips. I use Printfile sleeves for 4x3 frames. Some of the labs use 3-channel sleeves that are on a roll. They usually then cut the film 3:4:3 with the rebates on, and cut the sleeve to length. Both ways will allow you to use 9,5 x 12 " paper. If I were a client, I would not want to get a film back in single frames. It will make scanning more laborious, and there is a better chance of damaging a frame. It is also more difficult to sleeve and retrieve from the sleeve afterwards. So leave it like you have it now, and then use larger paper. If you don't find the ideal size, then use the next larger size trim it back with a guillotine. Another possibility is to buy roll, and cut to size with a guillotine. I am sure your client is worth more to you than skimping on photo paper.