As long as the bosses are there, drill three holes, tap three holes, done. Cannot take half-an-hour.
I have heard of--but never seen--pickup-truck blocks that have a step machined into the tops of the lifter bores, so that OEM roller lifters, dogbones, and spiders appear to fit but the lifters leak oil out the bores. If your lifter bores aren't stepped...you're good.
Hard to believe, unless you intend to buy all new parts. You'd need the cam, the lifters, dogbones, a spider, three bolts, and new pushrods. You'd want a timing set either way, but you need a different timing set. There's one available from GM that includes both sprockets, the chain, both the early design and the late design thrust plate, and the two bolts that hold the thrust plate. Not horribly expensive, either.
https://www.summitracing.com/parts/nal-12371043/overview/
When it was me, I paid ~$200 for a used Vortec short-block, got everything I needed except pushrods. (I re-used the cam and lifters after cleaning them, and I had to buy hardened pushrods to go with the guideplates on my aftermarket heads.) If you want an aftermarket cam, you'd add the cost of the camshaft, and maybe the cost of lifters.
Buying the spider, dogbones, lifters, cam, thrust plate and pushrods at a Pick-A-Part would be--what? $50? I don't know, we don't have pull-your-own Treasure Yards around here.
Small price to pay to get rid of flat tappets in an era of compromised oil. (The only thing you have in your favor is that the valve springs are not tremendously stiff.)
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