I would give Chris Weddle call. They have been doing rebuilds for over 25
years. They SPECIALIZE in building strong transmissions.  They can build you
a four or five speed box to easily handle your engines horse power.

I bought a five speed from them with a locking diff.  The locking diff acted
up and they removed the diff and gave me credit with NO problems.  They
shipped the tranny and UPS damaged it. I returned it and they upgraded my to
a brand new tranny with a .457 ring and pinion from a .483 R&P at no
additional charge.
They ended up giving me a brand new .457 tranny with 0 miles.

Robert