To me, it sounds like you are shifting too early.
And you have a totally wrong gear,
But the 99MPH tells the real story, you are way down on power.
My 360 is very similar to yours, and with 3.55s and at 3850(me in it),and a much smaller cam, went 106 with no tuning at all. Back in 1971,my totally stock 70 Swinger340 at 3110(me in it,IIRC), also with 3.55s but with a 4spd, went 99mph.To me that makes it look like you are making a little more power than a stock 1970 Swinger 340.More cuz I'm sure the raceweight of your 73 is more than 3110.
So the question is why are you so down. It has got to be in the tune.
--Taking it out, and to the dyno is a huge investment in time and money.They are gonna do a lot of things that you can do yourself.And there will be a lot of things that they won't do, that you will need to do, later.
--I would go back to the basics; compression test, leakdown test, and timing test. If she's got more MPH in her, these tests will prove it. Or not.Then make sure the secondaries are opening, and that the carb is not fat.Fat will just kill mph.
--The no tire-spin and 1.95 60fts tell the same story; at a stall speed of 2800, there is nobody home.This can be traced to low cylinder pressure,insufficient timing and slow secondaries or fat carb. So again, it points to the tune. The Dyno may not catch this, cuz they are concentrating on WOT fuel and timing, and trying to maximize those numbers.And they do a good job of that. But when you get it back, you still have to make it driveable.
--So as to the basics; For a strong launch at 2800, you need torque. The 284 ain't that soft at 2800, so Ima gonna guess the timing is lazy down there or the cylinder pressure is low, or the secondaries are not opening. So if it doesn't respond favorably to increased timing or faster secondaries, then the next step is a compression test. Actually, I would start with that.At least then you can rule out compression. Or not. 165psi will make a good launch. 120psi will be lazy. 140 can be made to work, but probably not with a 2800TC, or 3.55s. Less than 120 you are gonna need way more gear.Ima thinkin 4.57s will get you around 107 at 6000 with 28s. I'm not suggesting you run those, just that with 120psi, you are gonna need all the help you can get.
--Concentrate on getting the mph up, and the ET will follow.Once the MPH plateaus, try shifting it at a higher rpm.Go up 200 at a time, til it slows down, then back up the bus;100 at a time until it peaks. With a 727, it will want a different shift rpm for each gear. This is because the gears are not evenly spaced.(With your combo you might be able to trap in second gear;With 28s, 105mph is about 6500rpm.) Then try a different power-timing, again looking for a max mph.When you finally get that mph thing bugged out; then start tuning the launch.
BTW, I heard you say stuttering at the big end. Not sure what that means but as someone else asked, what are the plugs telling you? If the carb goes real fat, not all ignition systems can fire that. If it goes lean the plugs misfire, glaze up or break insulators.So learn to read plugs and always carry a spare set, until you get the hang of it.
And if you really want to do yourself a favor, run the carb on the primaries only, and get that dialed in first.Take your time, one change atta time. Don't melt it down. May the force be with you.
