you're absolutely right, going on that, if you want to stay with duraspark ignition, just run down to your local parts store, and grab a dizzy for an '85 GT vin code M (4BBL HO), that was the first, last, and only year the carb'd mustangs came with a roller cam. It will come freshly reman'd with new everything, including the steel gear for about $50 + your old core ($30)
if you wanna swap to a standard style roller cam, you have to buy the linkbar type lifters...which run about $400. If you want to use the stock type lifters, then yes, drill and tap the lifter valley to run the spider and dogbones, but you'll have to use a reduced base circle cam. those run about $300 from Comp. I'd thought about doing that swap in the past, and after running all the numbers it just was more advantageous for me to wait and get a new badass block (which I'm still doing), so I can use standard dimension parts.
The reason behind it is the lifter bores...the older style non-roller blocks have shorter bores than the roller style blocks.