58-62mph should be safe. . .the best 3,000rpm is Max on the GMC V-6 or big block V-8's inline 6's should hold 3250rpm and the small block v-8's can handle 3500rpm. . .

As a rule of thumb I use 3,000rpm as my max speed across the board. . .

Mike

1962 GMC K3000, GMC-379 V6.