This topic appears with such frequency I think it should be added to all amateur examinations. http://users.aol.com/rfelectron/faq.htm
I have no idea what type of amplifier (e.g. tube, solid-state, mobile, fixed base) you are considering for conversion to the 40 band.
My short answer is it MAY be useful for parts to build your own -- but this would be for your educational, learning and building enjoyment (e.g. It is NOT based on economics).
There are plenty of very good HF amplifier designs on eBay and for sale on a weekly basis at good prices -- that already work.
Look at eHam.net reviews and "check around" before purchasing any amplifier. A good Google.com search will quickly imform you for the preferred tubes (NOT sweep tubes) or solid-state models in the marketplace.
What makes a good solid-state amplifier? (Adam Farson actually has a web page for a bit of theory and to understand "Why Not?"http://www.qsl.net/ab4oj/quadra/sshfamp.html
What makes a good tube amplifier?http://www.qsl.net/kk5dr/goodamps.htm