I have a HobbyLab SR-71, and I have flown it on an Aerotech reloadable F12 and also an E18. These two reloads are for the regular 24/20-40 RMS casing which has the delay element cap with the ejection charge well. I assemble the reload per instructions, I simply don't put any black powder in the ejection well. Since you aren't really employing the ejection charge, it doesn't matter what delay time you use. BE SURE to use the delay element, I wouldn't substitute wadding or anything like that (not to mention I'm pretty sure that substituting wadding for a delay element would constitute illegal modification to a commercial engine at a sanctioned TRA or NAR launch).
VERY IMPORTANT - as George said in his post, make sure that you check the balance with whatever motor you plan on using. This bird is pretty sensitive to having the proper CG. Some of these birds were also bad about the radio components coming loose inside during boost or a rough landing, which can radically change the balance, which ends up being bad news.
I've had a couple of people tell me that they couldn't fit the regular 24mm AT RMS casing in their HL SR-71 motor mount, but it works fine in mine. Perhaps there were manufacturing variances that prevents that casing from working in some of them. If you decide to try this, I'd check the fit before going to the field.
The F12 is an interesting motor for the bird, as it leaves a long black smoke trail for the BlackBird. Looks neat. The E18 motor will come close to duplicating the performance of the single use E15PW Aerotech motor - they are very close as far as thrust characteristics.
It sounds like you have a fair amount of RC experience, and you say you flew the SR-71 with a pusher prop power plant (say that real fast 3 times). The only thing you want to remember is that with a glider, and particularly a delta variant glider, you can bleed speed really fast in a turn. I've had that happen with the HL SR-71. Pull a hard turn with it at low speed, and part way through the turn it's quite likely to stall and snap on you. To turn this bird, I always do one of two things - go into a shallow dive prior to the turn to build up sufficient speed, or make sure that I let it descend in the turn so that it will maintain speed. I don't recommend any sharp turns unless you have plenty of altitude.
It is a fun and interesting model to fly, and it always attracts attention at a launch.