.842s would be fine and are fine for 1.8 HP/CI builds, turbo and most blown set ups. but for a all out NA, the .842s would be a weak link for 2 reasons ,1) the largest wheel you can get in the .842s is a.750 DIA, the .750 wheel has a hard time dealing with the open spring pressure you would need , less surface area to spread out the load of the spring.
2nd reason is at this RPM 8K+ you need to hold the valve open as long as you can on the top of the lobe.....it's a similar to why all out flat tappet engines use .904-1.0 dia , look at the GM RO7/ROX and SB2 programs, Look at Australian pro-stock program, those engine are limited to 400CI and make can make over 1000HP, you won't find anything smaller than a .904 DIA roller lifter. so for a Max NA G3 we need to have a .800-.840 dia Roller, which you can only get in .904-.937s....+ it's easier to control the rate of closing with the larger wheel,
Steff pointed out that the 6.1 block and the BGE block(all Iron G3's) do not have enough room in the lifter bore to be enlarged enough to use a .937 Keyway with the bushing....I've used the .937 keyways with out, but It's not a street friendly idea....so that leaves us with the Modified link bar lifters....issue with those is there is not enough space between the heads and the block to use a link bar as is.I've used Comp cams LS short travel link bars in a 374 build, but the bottom of the heads(eagle)needed to be clearanced.
Answers are coming in the next year.

Now one of the benefits of the BGE and the 6.1 block is ….cam tunnel you can seal the cam tunnel off by taking a piece of Alum and attaching it to the oiler mounting boss.
Another advantage the G3 has is the nature of the crossflow head and the efficiently of the combustion chamber we should be able to hit 2.5 Hp per inch with less lift than a wedge or canted valve head...we just have to find that # and the RPMs