The MPICH-SCore high performance communication library for cluster computing is integrated into the MPICH- G2 library in order to adapt PC clusters to a Grid environment. The integrated library is called MPICH-G2/SCore. In addition, for the purpose of comparison with other approaches, MPICH-SCore itself is extended to encapsulate its network packet into a UDP packet so that packets are delivered via L3 switches. This extension is called UDP- encapsulated MPICH-SCore. In this paper, three implementations of the MPI library, UDP-encapsulated MPICH-SCore, MPICH-G2/SCore, and MPICH-P4, are evaluated using an emulated WAN environment where two clusters, each consisting of sixteen hosts,are connected by a router PC. The router PC controls the latency of message delivery between clusters, and the added latency is varied from 1 millisecond to 4 milliseconds in round-trip time. Experiments are performed using the NAS Parallel Benchmarks, which show UDP-encapsulated MPICH-SCore most often performs better than other implementations. However, the differences are not critical for the benchmarks. The preliminary results show that the performance of the LU benchmark scales up linearly with under 4 millisecond round-trip latency. The CG and MG benchmarks show the scalability of 1.13 and 1.24 times with 4 millisecond round-trip latency, respectively.