Looks to be a lot of meat in the upper parts of that compound. I'd look to milling out the Tee slot to give a flat top to fix the toolpost down onto using the usual stud. Might need to have a head on the bottom side of the stud if things are a bit thin for a simple thread to be secure. If it does come out a bit thin screw and bond a piece of 1/4 plate on for the post to sit on. If you still aren't happy with completely clearing the slot 1/16 to an 1/8 of metal loaded filler under the 1/4 plate where the bottom of the slot was will do just fine. Base of that 4 way can thin down to half its thickness.
Sounds like funds are too darn tight to cover a quality QC set and enough tool holders. If so you could simply make your own 4-way by screwing and gluing together suitable size square and plate stock. Two way is easier and just as good, yet to see anyone seriously use a fully loaded 4 way on an ordinary centre lathe, especially if you arrange a method of rapid interchange so you can keep a bunch ready loaded and just switch blocks. Tool shimming to centre is no great problem given a simple jig and dial gauge with elephants foot gauge and sorted stock of shims / spacers on the bench so you can set-up in the block by direct measurement off the machine. For carbide tooling you make the post so it comes right.
When I was planning a rapid interchange block system it seemed to me that a effective, simple, way of going about things would be to make the centre post freely rotating and cross drill it near the top for a tommy bar. A externally threaded castellated nut with apertures of the same size as the cross drilled hole would be arranged to screw into the top of the tool block. As the tommy bar passes through both nut castellations and cross drilled centre post it should be no great problem to arrange things so that around 1/3rd of a turn covers the range between tightly clamped down and loose enough to pull out the tommy bar so that the block can be lifted off and another substituted. Easy to arrange registration pins, ratchet locators et al underneath as seems fit so that the blocks always return to the same position.
Clive
