have a '73 240 with an L-28 and a new 5-speed. Runs through 1-4 just fine, but will slip out of 5th after 5-10 seconds unless I hold the shifter in place. have just replaced the clutch master and slave cylinder. Any ideas?

Could be two things; 1. Your 5 speed isn't new. And 2. Check to make sure that your shifter isn't hitting any part of the center console. I've heard that if you don't get the right shifter then it will bind, rub, or get obstucted by the center console which requires a little bit of cutting.

okay, it isn't a brand new 5-speed. It came out of some variety of 280. If it isn't the shifter not being able to make the throw, what could it be in the transmission, and am I looking at a rebuild, or a new transmission?

The inside's on your tranny are probably worn out (old and abused). There are several parts that could be worn causing it to pop out of gear. If you are going to have it rebuilt I guarantee you that fifth isn't the only gear that is going to need some work. I'm not sure if it would even be worth the trouble of rebuilding. Drain the fluid on the tranny and look at the plug. It has a magnet attached to it and should give you an indication of the metal shavings that have been floating around. Alot of shavings bad. Small metal chunks bad. By the way do you know who did the swap and did they change the fluid?

There are alot of other trannys out there. My suggestion is to get another one. If you plan on being abusive then get a tranny out of a turbo model. I believe these were made alot tougher to handle the power of the turbo.

You should be able to pick one up for 100-200 dollars. If you get one at a junk yard they usually give you a 30 day warranty. Which is plenty of time to find out if it is bad. Make sure you change the fluid. And before you take it home lay the tranny up side down pull the drain plug and inspect the magnet.

Good luck

Ed

thanks Ed, I appreciate your time and info.