Are the PRE Magnum roller engines worth building ?

Well, think about it for a second. The regular engines have Hyd. Cams. So swapping over is costly with the lifter expense. Then again, you may also want to move away from the factory lifters or you have none and may need to purchase new ones.

The blocks also have something the Magnums do not have and that would be a oiling hole to the heads to oil the rocker shafts. magnums do not have this provision. You'll have to (pay someone) drill the passages out. It is unmarked.

The heads are OK and fine. 302 or the 308. IMO, there a decent OE head not to go bonkers for. Nor any OE head.

In short, your answer is YES