yes, you do need to maintain the correct angles. The trans doesn't care what angle it is at as long as the fluid level stays high enough for the pickup tube to suck it up. The most important angle to have correct is the driveshaft angle. If I recall, you want to keep the angle on each end of the driveshaft as close to the same as you can. check out this picture: http://www.jalopyjournal.com/forum/attachment.php?attachmentid=1194679&d=1293946775
A TH350 is very easy to mount, as for the trans itself, the only thing that you will have to worry about is the shifter linkage, if you use a cable, it makes it alot easier.
An angle finder can help you alot, you can get them at any of the import stores pretty cheap, i think i paid $7 for mine. they look like this:
http://www.harborfreight.com/dial-gauge-angle-finder-34214.html
hope that helps,

jason

1937 GMC T16 468 BBC th350, 4.10
1941 Dodge COE 5.9 cummins, 9 speed roadranger
1947 Diamond T COE, hercules and 4 spd, all original
1955 Chevy LCF 9.0L IH diesel AT545
1962 Corvair (Elsie)
1967 Jeepster Commando
1968 K10
1970 Chevy C50 box truck