Do you think the off brand rear blocks from Amazon are OK? Also, are they supposed to be tapered?

2" rear blocks

 

They look fine to me. The taper helps the leaf springs sit at a natural position and gets them seated properly at a good angle. There are completely flat blocks that do fine as well but rely more on the torquing of the U-bolts to seat the leaf springs adequately to the block. May or may not make a difference, but the taper makes a lot more sense to me.

 

Now you did notice the amount of lift to expect with this set correct. With a 4x4 you will gain approx. 1/2" of lift, but on a 2wd you will gain 2" of lift. Not sure what your going for.

On the other hand, I have purchased a couple of set of rear blocks off of Amazon and eBay that look like this set posted. They have all be well built, welded and complete.

 

I am wanting to add about 1 inch lift in the rear. I might have to go with a 3 inch block?

Right now my rear fender is 39.25 to the floor and my front is 37.75 which is a 1.5 inch difference.
I am wanting to add front Billstein 5100's set at 1.25 which would make it a .25 rake.
I tow occasionally tow a trailer with a 300# tongue weight so I am wanting to keep some more rake for rear sag.

 

Measure whatever Block you have in the Rear, Stock, then add to that, how much you want to Lift, that'll give you the number of what size block you'll need.

IE: If you have a 1" Block from the Factory, and you wanna lift it 2", you'll need a 3" Block.

The Taper in the Block does absolutely nothing for the Leaf Springs. It "Rolls" the Axle to be at the correct angle so you don't get driveline vibration. The Higher you go, the more of a Taper you'll need.

DO NOT STACK BLOCKS EITHER.

 

I would like it to be almost level when not towing and not sagging when towing. Will a Roadmaster Active Suspension work for that?

 

For what it is worth, you should probably get the front end work completed then give it a day or 3 to settle then measure to see what you need.
The 1.25" 1) may not raise it as much as you'd wish it had and you will need to adjust the rear accordingly 2) may not be the exact 1.25" that you are counting on by purchasing rear setup in advance.

Drive the truck home with the new front end. Measure. Add the blocks to your Amazon cart but do not buy.
The following day, drive again just so it jostles around, and is driven. Measure. If same measurement as the day before, open your cart and purchase, or adjust as necessary.
If you have any ballast weight, throw in a few hundred pounds to see how much the rear settles. Might be useful to know just how much sag you will see during towing, to add to your math homework.

 

I just put these on a couple weeks ago. Good quality blocks, well packaged, decent instructions. I can't comment on how much lift you'd get but, I got somewhere like .75" with measurements afterwards.

 

If I were to do what you're asking, that RAS would be the way for me to go. It can be adjusted to do exactly like you want. Easy assembly. Just my opinion.