The FRST block uses a Crank Position Sensor in it which requires a blanking plate or similar. You need to remove the push fitted oil return as the FRST one (if still on a T2) is of the threaded type which doesn't suit the T3's oil return hose (cost all of 97p from Ford when I changed mine).

Heads are totally different though and not really interchangable without adapter plates as the inlet manifolds and injection systems are different.