Short answer is, Holley moves the motor forward while TTI and Schumacher do not. The difference between TTI and Schumacher are pretty minor beyond that.
Not sure this is definitive but hope it helps.
Holley:
1. '73+ V8 K-Frame only
2. Moves the motor forward 1.75" which allows for room for the SRV module, OEM low mount AC compressor and a 90 degree oil filter but messes up the trans mount and linkage
3. Headers are shorty only but allow for Borgeson PS, stainless and are about $300 cheaper than coated LT TTI's or about $175 cheaper than the polished shorty TTI's. These probably only work with Holley's mounts too.
TTI
1. Can do biscuit or spool mounts but V8 k-frames only
2. Keeps the motor in the stock location
3. Requires a remote oil filter and firewall notching for the SRV module (or a lockout)
4. Shorty or LT headers available and Borgeson PS an option
5. Can not use the OEM low mount AC compressor, mounts are used by the DS motor mount but their notes are confusing
Schumacher
1. Can do biscuit V8 or /6 k-frames or spool mount V8 k-frames
2. Keeps the motor in the stock location
3. Requires a remote oil filter and firewall notching for the SRV module (or a lockout)
4. No idea on header options, but I suspect they use TTI headers
5. Can not use the OEM low mount AC compressor per their notes
