The issue is the height of the lifer bores in the block. When Ford developed the roller lifter, they essentially grafter a roller wheel on to the bottom of flat lifter - makeing it significantly taller. So, the cast the new block with a taller lifter bore. Problem solved.
You really have two easy options. You can use a standard cam, and link bar lifters. Kind of expensive, as the lifters run about $500 a set. But you have a greater selection of cams.
You can use stock Ford lifters, dog bones, and spider. And then use a reduced base circle cam. Somewhat cheaper, but just as effective.
Installing the spider in the lifter galley is pretty simple. Drill and tap two small holes. Do not do this with the cam bearings installed, as you could damage them if you drill too deep.
The dog bones are used to keep the lifters from rotating. Flat tappets need to rotate, or they develop flat spots. Roller lifters need to keep the roller wheel aligned with the cam. The dog bones need to sit flat on top of the lifter bore. Sometimes you need to do a little grinding to get everything to fit right. On my stock block, it only took a couple of minutes with a 4" grinder.
I would not choose to install a flat tappet cam in any windsor motor. It's too easy to convert to rollers, and there are some excellent benefits to be had.