He is going to put IRS under his TR3A ?? Lotta work there, starting with some major frame fabrication. Be easier IMO to put a TR3A body onto a TR4A frame (ala TR3 beta).
I don't know how many TR6 came from the factory that way, but 4.10 gears are not hard to find. Try BFE or TSi. Carsten Conrad (in Germany) used to sell them as well, not sure if he is still in the business. Same (early) gear set will fit either solid axle or IRS diff.
However, my TR3 came to me with 4.10 gears, and I have to say they are way too short, especially without overdrive. I have to be careful not to spin the rear wheels, and even at 6000 rpm, the rpm is limiting top speed.
Steve Hedke did a careful study with 'Scrappy' (the TR3 he ran in The Great Race
https://www.fot-racing.com/tr3/caption/TR3Hedke.htm) and found that he actually got better acceleration with 3.45 gears!
TR4 tranny is no problem. Just elongate the holes in the rear crossmember, to move the rear mount back by about 3/8". If the body is pre-TS50k, he'll need to rework the trans tunnel a bit to fit over the starter bulge. Personally I prefer the TR3 shift lever but the TR4 lever will work.
PS, TRF has 4.10 gear sets on sale for $500 at the moment, PN 505014