I think you are fine.
I ran a Precision .96 AR T4 flange 7675 CEA wheel for a while on an ON3 and then on a Ponydown system.
It made around 1000 HP+ on my 363 at 25 psi and spooled instantly. I ran right at 5.40's at 130 MPH 1/8th mile in 3250 lbs with full dual exhaust. The 3.5" downpipe split to dual 3" to Borla pro XS. That turbo dyno'ed 1200, but that was on a Dynojet. They seem to read high, but the ET and MPH was good. Powerglide transmission, 1.68 low gear, 4.10 rear gear.
I'm now running a T4 from Forced Inductions with a ported 1.25 A/R turbine housing. I use a modified B&G hotside. I bought the headers from them and made my own turbo piping, I still use a single RG-45 gate placed right below the turbo flange in my system. I tried a big fancy expensive gate, but the RG45 worked much better so I put it back on.
My system is good with the single RG-45, but I put it in a better spot than most kits. I can dial any boost I want from 9 psi (spring) to 30 psi.
My car is exactly 3200 lbs race ready and goes right at 150MPH in the 1/8th through exhaust to the rear axle. It has a lot of backpressure, but still makes good power. This is actually the real exhaust flow limit for the T4 1.25 AR, so a good T4 is nowhere near as power limiting as most people think.
I really doubt you need to go to a T6. A T6 will a lot bigger, heavier, and a Precision 7675 with a CEA wheel would easily go 1000 with the turbine housing you have if your engine is decent.
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89 LX. 363, single turbo, Super Vic EFI, TFS high port heads by TEA, solid roller, glide. Holley HP EFI. (exact combo varies)