The differance between a roller and a non-roller block is the hight of the lifter boss. That's because the Ford replacement roller lifters are taller than flat tappets.
If you simply drop roller lifters in the non-roller bores, they will stick up so high that they'll starve for
oil and die rather quickly. The other issue is that flat tappets are supposed to spin in their bores. But rollers have to be perfectly aligned at all times.
The simplest change is to use a reduced base circle cam, and stock ford lifters and retainers - spider and dog bones. You'll need to to make two small changes to your block. First, you'll need to drill and tap two 1/4" holes in the galley. They only need to be about 1/2" deep. They are directly over the cam bearings, so don't drill too deep. You don't need the bosses cast into the block. You may also need to clearance the galley around some of the lifter bores so the dog bones work correctly. Both of these small jobs can be done with the assembled engine. Just be carefull about chips and shavings.
Here's what it looks like installed in a stock '74 351W block
Here's a close up of the lifters. You can see where I clearanced the block for the dogt bones. It just needed a slight touch with a grinding wheel.
Some people claim the reduced base circle cam is not as good as a standard cam, mostly because the core is thinner. With heavy valve springs, the cam can twist and change the valve timing at the rear of the motor. Unless you're doing a lot of high rpm racing, I don't think that's an issue. But if you were doing that, you wouldn't be using this set up.
The other method is to use a standard cam - with a standard base circe - and link bar lifters designed specifically for this conversion. This is a little easier, but a lot more expensive. The lifters are about $500-600 a set. Also, since they're taller, they heads have to come off to remove them. So if one fails, or you want to swap the cams.......
In my Dart block I use stock modified Ford lifters, dog bones, and spider.