You don’t need an LSD. My buddy is going 10.7 @ 132 on the stock 1st gen trans and axels. I went 12’s on stock trans, open diff.
If you’re only going 74 in the 1/8th, you need to turn up the boost, lol. I was going 93 in the 1/8th, on the big turbo, and like 88 - 89 (if my memory serves me correctly) with the SRT turbo.
I would find out why your spark is cutting out at 6k, and confirm that it is spark cutting out. What are your plugs gapped at, could they be blowing out? I’d troubleshoot, fix what you have, and then go from there.
And get those 2.5 60’s down!!!
–mark