What is TTI? I never heard of that.
What is the impeller speed of the 5hp MTD? It might be a better base impeller speed to start from than an older Ariens. MTD probably had 10hp models avaiable using the same 5hp frame with small tires but it would be worth checking some drawings and the MTD site or partstree.com.
Below is an ST824 924050 it is more modern that the 10,000 series and the bulk of prior 924's. BTW this is one I just rebuilt with 1 coat of undercoat and two top coats, rebuilt auger gearbox, and new flange and ball bearings in the bucket and tractor
The 924's has mostly 1080 impeller RMP and 108 auger RPM. Not the best for distance. The factory rating for these models was 3-30 feet and that covered up through 11hp. I put a new 11hp OHV on ths same model last year. It threw well and could do 30-35 feet in good conditions of eastern Mass. Note the forward tilt of the chute back angle. The second picture shows the low chute crank which an OHV head can go over. These bodies took at least 11hps from the factory. Not sure about 13hp.
The 924 series and 10,000 splatter their output so not the best if you want to reliably throw 25-30 feet and not get anything below that say in an area you just cleared. For distance I'd start with a better base and tweak that for RPM and put on a better chute. I'm currently fixing up an older 10hp Snapper fitted with newer tall slow back angle John Deere chute but I don't expect a big improvement even with that great chute.
You could probably tell your auger RPM by putting a piece of tape on an auger and counting rotations for 10 seconds like a nurse does your heart beat. I've been thinking about this for some time so will give it a shot tomorrow.
A good base to start from would be a 624 Yamaha body. They come up used all the time. It might throw well enough to meet your requirements as is. I tossed in the same conditions with a Yamaha 624 and an Ariens 1028 with a newly rebuit engine with 5% leakdown compression. The Airens just barely beat the 624.
What do you think your getting for distance with the 8hp? Where are you, in dry or heavy snow country?