Go with a double bevel. Weld the first side, back gouge and weld the second side.
Do you have a copy of D1.4? I'm traveling, so I can't do the leg work for you.
Make sure you have CMTRs for the rebar received so you know what you are dealing with. If the sulfur is more than 0.04%, make sure you use a high manganese filler metal. Compare the deposited weld metal chemistry for several manufactures to get the right balance of manganese. The manganese should be 10 times the sulfur content. You may have to specify a specific manufacturer and trade name for the filler metal to get what you want. Not all electrodes are created equal.
For preheat; use the higher of either D1.4 or the calculated preheat using the annex of D1.1. If you elect to use the annex of D1.1 assume you have medium restraint condition and a H2 condition if you are using SMAW (EXX18) or FCAW with low hydrogen characteristics. Not all FCAW electrodes meet the low hydrogen requirements, i.e., 16 ml [H] /100 g of weld deposit. Even the standard H16 may be too high for the material you're working with. The strength of the filler metal can match the strength of the lower strength material being joined. You will have to qualify the procedure, so you will be able to verify the minimum tensile strength is met. I would weld the rebar to a plate, one rebar to each side of the plate so that you can evaluate the joint detail, filler metal, etc.
Best regards - Al